Objective Lens Selection Guide
Objectives
Super Apochromatic Microscope Objectives Microscopy Objectives, Dry Microscopy Objectives, Oil Immersion Physiology Objectives, Water Dipping or Immersion Phase Contrast Objectives Long Working Distance Objectives Reflective Microscopy Objectives UV Focusing Objectives VIS and NIR Objectives
Scan Lenses and Tube Lenses
Scan Lenses F-Theta Scan Lenses Infinity-Corrected Tube Lenses

Webpage Features
	Zemax black box files (both directions) for all tube lenses except the ITL200 can be accessed by clicking this icon below.

Tube Lens Options
Tube Lens Type	Effective Focal Length	Available Wavelength Ranges
Widefield	200 mm	UV
		Visible
		Visible and NIR
		NIR
	180 mm	Visible
	165 mm
	100 mm
Laser Scanning and Widefield	600 mm	Visible
	400 mm
	300 mm
	200 mm	Visible
		Visible and NIR
		NIR

Click for Details
Widefield imaging tube lenses take the collimated light from an infinity-corrected microscope and focus it onto the sensor of a camera, as shown on the left. Laser scanning tube lenses can be used in telecentric systems to scan a laser spot across a sample.

Features

• Tube Lenses for Widefield Imaging
  
  • True Imaging Lenses for Forming a Well-Corrected Infinity Optical System
  • Apochromatically Corrected for Diffraction-Limited Performance Across the Field of View
  • Better Overall Aberration Correction than Standard Achromats
  • 200 mm, 180 mm, 165 mm, or 100 mm Focal Length
• Telecentric Tube Lenses for Laser Scanning and Widefield Imaging
  
  • Infinity-Corrected Design
  • Forms a Telecentric System When Paired with an SL50 Scan Lens
  • 600 mm, 400 mm, 300 mm, or 200 mm Focal Length
• AR Coatings for High Transmission Through UV, Visible, and NIR Wavelength Ranges
• Compatible with Thorlabs, Nikon, Leica, Mitutoyo, Olympus, and Zeiss Objectives
• Diffraction-Limited Optical Performance

These infinity-corrected tube lenses are designed for use with infinity-corrected objectives from all major manufacturers, including the dry, oil immersion, and physiology microscope objectives sold by Thorlabs. Designed for high-resolution imaging, biomedical, machine vision, and laser scanning applications, these lenses can be aligned in pairs to create relays, combined with objectives to create different effective magnification ratios at a scientific camera, used as drop-in replacements for tube lenses in existing systems, or integrated into DIY Cerna^® Microscopes and other home-built microscopy setups to generate high-quality images.

Standard Widefield Tube Lenses
Our widefield tube lenses are AR-coated for high transmission and provide diffraction-limited axial color performance for the UV, visible, and NIR wavelength ranges; see the Specs tab for performance data. Their effective focal lengths correspond to the design focal lengths of popular objectives (see the Magnification & FOV tab for details). The TTL200 series of tube lenses are specifically designed to offer a wider diffraction-limited axial color range than the ITL200 tube lens. Specifications for all lenses can be found on the Specs tab, as well as Zemax black box files for all lenses except the ITL200.

Telecentric Tube Lenses for Laser Scanning and Widefield Imaging
Our laser scanning tube lenses are optimized for laser scanning applications, such as confocal laser scanning, two-photon microscopy, and three-photon microscopy. These lenses create a telecentric system when paired with our SL50-CLS2, SL50-2P2, and SL50-3P scan lenses, for use in point-by-point galvo scanning of the object plane. These lenses can also be used for widefield imaging over their specified wavelength ranges.

Our standard widefield tube lenses for the visible wavelength range can also be used for laser scanning purposes when paired with the CLS-SL visible scan lens for example. However, using a standard tube lens in a scanning configuration will limit the unvignetted field size, since the tube lens must be placed at the telecentric pupil distance from the objective (e.g., 250 mm for the TTL200 lens), rather than the intended pupil distance of the tube lens.

Microscope and Objective Optical Compatibility
Microscope manufacturers design their systems with one of several standard tube lens focal lengths, including 200 mm (typical for Thorlabs, Nikon, Leica, and Mitutoyo microscopes), 180 mm (typical for Olympus microscopes), and 165 mm (typical for Zeiss microscopes). Similarly, microscope objectives are designed to provide the magnification engraved on the housing when used with a tube lens of the appropriate standard focal length. We offer infinity-corrected tube lenses in all of these focal lengths so that home-built microscope systems may make use of these industry standards.

Alternatively, objectives and tube lenses with different design focal lengths may be combined to create different magnification ratios at the camera without compromising the axial color correction. We offer four tube lenses with non-standard focal lengths that can be used to change the magnification of an existing system. To calculate the system magnification for different tube lens and objective combinations, see the Magnification & FOV tab.

Tube Lenses for Widefield Imaging

Item #	TTL200-UVB	TTL200	TTL200-A	TTL200-S8	TTL200-B	ITL200	TTL180-A	TTL165-A	TTL100-A
Effective Focal Length	200 mm ± 1%	200 mm ± 1%				200 mm	180 mm ± 1%	165 mm ± 1%	100 mm ± 1%
Working Distancea,b	176.4 mm	148.6 mm	151.8 mm			148 mm	133.5 mm	120.6 mm	63.1 mm
Pupil Distancec	50 - 80 mmd	70 - 170 mm				70 - 170 mm	50 - 150 mm	50 - 150 mm	0 - 100 mm
Field Size at Image Plane	Ø24 mm	Ø22 mme				Not Available	Ø22 mme	Ø22 mme	Ø15 mme
Entrance Pupil	Ø13 mm (Max)d	Ø20 mm			Ø22 mm	Not Available	Ø18 mm	Ø16 mm	Ø14 mm
Lens Design	Apochromatic	Apochromatic				Apochromatic	Apochromatic	Apochromatic	Apochromatic
Design Wavelength Rangeb	248 - 700 nm	400 - 750 nm	400 - 750 nm	400 - 2000 nm	650 - 1050 nm	Visible Wavelengths	400 - 750 nm	400 - 750 nm	450 - 750 nm
AR Coating Range	240 - 360 nm	350 - 700 nm		Broadband Single-Layer MgF2 Coating	650 - 1050 nm	Visible Wavelengths	350 - 700 nm	350 - 700 nm	350 - 700 nm
Axial Color	Diffraction Limited	Diffraction Limited				Not Available	Diffraction Limited
Resolution	Diffraction Limitedf	Diffraction Limitedf				Not Available	Diffraction Limitedf
Surface Quality	20-10 Scratch-Dig	60-40 Scratch-Dig				Not Available	60-40 Scratch-Dig
External Threading	M32 x 0.5 (Top) SM1.5 (Top and Bottom)	M38 x 0.5 Bottom Only	SM2 Top and Bottom			M38 x 0.5 Bottom Only	SM2 Top and Bottom	SM2 Top and Bottom	SM2 Top and Bottom
Housing Length	49.5 mm	28.0 mm				28.0 mm	31.1 mm	30.9 mm	33.5 mm
Performance Data (Click for Graph)
Transmission
Axial Color		400 - 800 nm 650 - 1100 nm 1000 - 2000 nm				Not Available
RMS Wavefront Error						Not Available
MTF						Not Available
Distortion						Not Available
Data	Excel Spreadsheet	Excel Spreadsheet				Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet
Zemax Black Box Files	Forward Backward	Forward Backward	Forward Backward	Forward Backward	Forward Backward	Not Available	Forward Backward	Forward Backward	Forward Backward

• Measured from the top edge of the housing to the image plane (see the diagram below).
• TTL series lenses are designed to provide diffraction-limited performance from 240 to 700 nm (for the TTL200-UVB) or from 400 to 2000 nm (for all other TTL series lenses); however, the working distance is valid only over the specified design wavelength range. The lens will need to be refocused when used at other wavelengths. Once refocused for a new center wavelength, the performance will be diffraction limited for some finite bandwidth around that wavelength. Nominal bandwidth, working distance, and performance can be estimated using the Zemax black box files. The axial color plots and performance data can be used for guidance when adjusting the working distance.
• This is the optimal distance between the tube lens and the entrance pupil of the objective (see the diagram below).
• The distance between the tube lens and the objective can be increased when using an objective with a smaller entrance pupil. For example, diffraction-limited performance can be achieved using the TTL200-UVB with a pupil diameter of 7 mm at pupil distances from 50 to 130 mm.
• The lens will provide an unvignetted field up to this diameter so long as the lens is focused at the wavelength of interest, which may be anywhere from the lower end of the design wavelength range up to 2000 nm. For reference, a Ø22 mm field size is large enough to fill a 4/3" format camera sensor.
• Appropriate for use with camera pixel sizes down to 2 µm.

Click To Enlarge
This schematic shows the working distance and pupil distance for the TTL series and ITL200 tube lenses. The working distance corresponds to the distance from the top surface of the housing to the image plane. The pupil distance, defined as the distance between the bottom edge of the tube lens housing and the entrance pupil of the objective, can be set anywhere within the range specified in the table above, since the rays from the objective are in parallel bundles. If the tube lens is too close or far, the image may suffer from aberrations.

Tube Lenses for Laser Scanning and Widefield Imaging

Item #	TL600-A	TL400-A	TL300-A	TTL200MP	TTL200MP2	TL200-CLS2	TL200-2P2	TL200-3P
Effective Focal Length	600 mm	400 mm	300 mm	200 mm
Working Distancea	94.2 mm			151.4 mm		180.9 mm
Pupil Distanceb	123 mm (Telecentric)			228 mm (Telecentric)		189.1 mm (Telecentric)
Field Size (Diffraction Limited)	Ø22 mm					16.3 mm x 16.3 mm (FN23) for 656.3 - 1100 nm; 14.1 mm x 14.1 mm (FN20) at 587.6 nm; 7.8 mm x 7.8 mm (FN11) at 486.1 nm	15.5 mm x 15.5 mm (FN22) for 680 - 1300 nm	15.5 mm x 15.5 mm (FN22) for 900 - 1600 nm; 12.4 mm x 12.4 mm (FN 17.6) at 1900 nm
Track Length	420 mm (Nominal)					409 mm (Nominal)
Entrance Pupil	Ø20 mm
Lens Design	Apochromatic
Design Wavelength Range	400 - 700 nm			400 - 2000 nm		450 - 1100 nm	680 - 1300 nm	900 - 1900 nm
AR Coating Range	400 - 700 nm			400 - 1300 nm	Broadband Single-Layer MgF2 Coating	450 - 1100 nm	680 - 1300 nm	900 - 1900 nm
f/#	30	20	15	10
Clear Aperture	Ø30.7 mm			Ø36.8 mm		Ø47.0 mm
Axial Color	Diffraction Limited
F-Theta Distortion	<0.5%	<0.2%				<0.3%
Threading	External SM2 Threads (Top and Bottom)					Internal SM2 Threads on Top External SM2 Threads on Bottom
Housing Length	205.7 mm			44.1 mm		41.6 mm
Performance Data (Click for Graph)
Transmission
Axial Color
RMS Wavefront Error
MTF
Data	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet	Excel Spreadsheet
Zemax Black Box Files	Forward Backward	Forward Backward	Forward Backward	Forward Backward	Forward Backward	Forward Backward	Forward Backward	Forward Backward

• Measured from the housing edge to the intermediate image plane (see the diagram below).
• This is the optimal distance between the lens and the objective (see the diagram below).

Click To Enlarge
This schematic shows the TTL200MP Tube Lens in a laser scanning configuration with the working distance and pupil distance called out. The working distance corresponds to the distance from the edge of the housing below the engraving to the intermediate image plane. The pupil distance is defined as the distance between the edge of the tube lens housing above the engraving and the entrance pupil of the objective.

#	Product Description	Qty.	Photo (Click to Enlarge)
1	Microscopy Camera	1
2	SM1A9 - Adapter with External C-Mount Threads and Internal SM1 Threads	1
3	SM1ZM - Non-Rotating SM1 Zoom Housing for Ø1" Optics	1
4	SM1L03 - SM1 Lens Tube	1
5	SM1A2 - Adapter with External SM1 Threads and Internal SM2 Threads	1
6	SM2L39 - Lens Tube Spacer for Tube Lenses	1
7	TTL200-A - Widefield Imaging Tube Lensa	1
8	SM2A59 - Adapter with Male D1N Dovetail and Internal SM2 Threads	1

• The adjustable C-mount to SM2 camera tube can also be constructed using the TTL200-S8, TTL200-B, TTL200MP, and TTL200MP2 tube lenses.

Adjustable C-Mount to SM2 Camera Tube for Widefield and Telecentric Tube Lenses

In the following example, an adjustable C-mount to SM2 camera tube is created to attach a custom widefield imaging assembly to a DIY Cerna microscope. An SM1A9 Adapter (2) is used to connect a C-mount Microscopy Camera (1) to the SM1ZM Zoom Housing (3). An SM1L03 Lens Tube (4) provides extra thread depth to allow full travel of the zoom housing, and an SM1A2 Adapter (5) converts the SM1 threads to SM2 threads. An SM2L39 Lens Tube Spacer* (6) positions the camera at the appropriate working distance from an attached infinity-corrected Tube Lens or Telecentric Tube Lens (7). The SM2A59 Adapter (8) enables the optical assembly to be connected to a DIY Cerna Microscope via a D1N dovetail.  

Components used in the configuration pictured below are listed in the table to the right. These parts, along with optomechanical components used to construct a DIY Cerna microscope, are sold separately.

*The lens tube is designed to positon the camera at the image plane of the tube lens when the SM1ZM Zoom Housing is at the center of its adjustment range, allowing for ±1.75 mm travel.

Click to Enlarge
An adjustable C-mount to SM2 adapter can be constructed to attach custom widefield imaging assemblies to DIY Cerna microscopes. 

When viewing an image with a camera, the system magnification is the product of the objective and camera tube magnifications. When viewing an image with trinoculars, the system magnification is the product of the objective and eyepiece magnifications.

Magnification and Sample Area Calculations

Magnification

The magnification of a system is the multiplicative product of the magnification of each optical element in the system. Optical elements that produce magnification include objectives, camera tubes, and trinocular eyepieces, as shown in the drawing to the right. It is important to note that the magnification quoted in these products' specifications is usually only valid when all optical elements are made by the same manufacturer. If this is not the case, then the magnification of the system can still be calculated, but an effective objective magnification should be calculated first, as described below.

To adapt the examples shown here to your own microscope, please use our Magnification and FOV Calculator, which is available for download by clicking on the red button above. Note the calculator is an Excel spreadsheet that uses macros. In order to use the calculator, macros must be enabled. To enable macros, click the "Enable Content" button in the yellow message bar upon opening the file.

Example 1: Camera Magnification
When imaging a sample with a camera, the image is magnified by the objective and the camera tube. If using a 20X Nikon objective and a 0.75X Nikon camera tube, then the image at the camera has 20X × 0.75X = 15X magnification.

Example 2: Trinocular Magnification
When imaging a sample through trinoculars, the image is magnified by the objective and the eyepieces in the trinoculars. If using a 20X Nikon objective and Nikon trinoculars with 10X eyepieces, then the image at the eyepieces has 20X × 10X = 200X magnification. Note that the image at the eyepieces does not pass through the camera tube, as shown by the drawing to the right.

Using an Objective with a Microscope from a Different Manufacturer

Magnification is not a fundamental value: it is a derived value, calculated by assuming a specific tube lens focal length. Each microscope manufacturer has adopted a different focal length for their tube lens, as shown by the table to the right. Hence, when combining optical elements from different manufacturers, it is necessary to calculate an effective magnification for the objective, which is then used to calculate the magnification of the system.

The effective magnification of an objective is given by Equation 1:

(Eq. 1)

Here, the Design Magnification is the magnification printed on the objective, f_{Tube Lens in Microscope} is the focal length of the tube lens in the microscope you are using, and f_{Design Tube Lens of Objective} is the tube lens focal length that the objective manufacturer used to calculate the Design Magnification. These focal lengths are given by the table to the right.

Note that Leica, Mitutoyo, Nikon, and Thorlabs use the same tube lens focal length; if combining elements from any of these manufacturers, no conversion is needed. Once the effective objective magnification is calculated, the magnification of the system can be calculated as before.

Example 3: Trinocular Magnification (Different Manufacturers)
When imaging a sample through trinoculars, the image is magnified by the objective and the eyepieces in the trinoculars. This example will use a 20X Olympus objective and Nikon trinoculars with 10X eyepieces.

Following Equation 1 and the table to the right, we calculate the effective magnification of an Olympus objective in a Nikon microscope:

The effective magnification of the Olympus objective is 22.2X and the trinoculars have 10X eyepieces, so the image at the eyepieces has 22.2X × 10X = 222X magnification.

Sample Area When Imaged on a Camera

When imaging a sample with a camera, the dimensions of the sample area are determined by the dimensions of the camera sensor and the system magnification, as shown by Equation 2.

(Eq. 2)

The camera sensor dimensions can be obtained from the manufacturer, while the system magnification is the multiplicative product of the objective magnification and the camera tube magnification (see Example 1). If needed, the objective magnification can be adjusted as shown in Example 3.

As the magnification increases, the resolution improves, but the field of view also decreases. The dependence of the field of view on magnification is shown in the schematic to the right.

Example 4: Sample Area
The dimensions of the camera sensor in Thorlabs' previous-generation 1501M-USB Scientific Camera are 8.98 mm × 6.71 mm. If this camera is used with the Nikon objective and trinoculars from Example 1, which have a system magnification of 15X, then the image area is:

Sample Area Examples

The images of a mouse kidney below were all acquired using the same objective and the same camera. However, the camera tubes used were different. Read from left to right, they demonstrate that decreasing the camera tube magnification enlarges the field of view at the expense of the size of the details in the image.

Click to Enlarge
Acquired with 1X Camera Tube (Item # WFA4100)

Click to Enlarge
Acquired with 0.75X Camera Tube (Item # WFA4101)

Click to Enlarge
Acquired with 0.5X Camera Tube (Item # WFA4102)

Damage Threshold Data for Thorlabs' Tube Lens for UV Wavelengths

The specifications to the right are measured data for Thorlabs' TTL200-UVB Tube Lens.

Laser Induced Damage Threshold Tutorial

The following is a general overview of how laser induced damage thresholds are measured and how the values may be utilized in determining the appropriateness of an optic for a given application. When choosing optics, it is important to understand the Laser Induced Damage Threshold (LIDT) of the optics being used. The LIDT for an optic greatly depends on the type of laser you are using. Continuous wave (CW) lasers typically cause damage from thermal effects (absorption either in the coating or in the substrate). Pulsed lasers, on the other hand, often strip electrons from the lattice structure of an optic before causing thermal damage. Note that the guideline presented here assumes room temperature operation and optics in new condition (i.e., within scratch-dig spec, surface free of contamination, etc.). Because dust or other particles on the surface of an optic can cause damage at lower thresholds, we recommend keeping surfaces clean and free of debris. For more information on cleaning optics, please see our Optics Cleaning tutorial.

Testing Method

Thorlabs' LIDT testing is done in compliance with ISO/DIS 11254 and ISO 21254 specifications.

First, a low-power/energy beam is directed to the optic under test. The optic is exposed in 10 locations to this laser beam for 30 seconds (CW) or for a number of pulses (pulse repetition frequency specified). After exposure, the optic is examined by a microscope (~100X magnification) for any visible damage. The number of locations that are damaged at a particular power/energy level is recorded. Next, the power/energy is either increased or decreased and the optic is exposed at 10 new locations. This process is repeated until damage is observed. The damage threshold is then assigned to be the highest power/energy that the optic can withstand without causing damage. A histogram such as that below represents the testing of one BB1-E02 mirror.

The photograph above is a protected aluminum-coated mirror after LIDT testing. In this particular test, it handled 0.43 J/cm² (1064 nm, 10 ns pulse, 10 Hz, Ø1.000 mm) before damage.

According to the test, the damage threshold of the mirror was 2.00 J/cm² (532 nm, 10 ns pulse, 10 Hz, Ø0.803 mm). Please keep in mind that these tests are performed on clean optics, as dirt and contamination can significantly lower the damage threshold of a component. While the test results are only representative of one coating run, Thorlabs specifies damage threshold values that account for coating variances.

Continuous Wave and Long-Pulse Lasers

When an optic is damaged by a continuous wave (CW) laser, it is usually due to the melting of the surface as a result of absorbing the laser's energy or damage to the optical coating (antireflection) [1]. Pulsed lasers with pulse lengths longer than 1 µs can be treated as CW lasers for LIDT discussions.

When pulse lengths are between 1 ns and 1 µs, laser-induced damage can occur either because of absorption or a dielectric breakdown (therefore, a user must check both CW and pulsed LIDT). Absorption is either due to an intrinsic property of the optic or due to surface irregularities; thus LIDT values are only valid for optics meeting or exceeding the surface quality specifications given by a manufacturer. While many optics can handle high power CW lasers, cemented (e.g., achromatic doublets) or highly absorptive (e.g., ND filters) optics tend to have lower CW damage thresholds. These lower thresholds are due to absorption or scattering in the cement or metal coating.

Pulsed lasers with high pulse repetition frequencies (PRF) may behave similarly to CW beams. Unfortunately, this is highly dependent on factors such as absorption and thermal diffusivity, so there is no reliable method for determining when a high PRF laser will damage an optic due to thermal effects. For beams with a high PRF both the average and peak powers must be compared to the equivalent CW power. Additionally, for highly transparent materials, there is little to no drop in the LIDT with increasing PRF.

In order to use the specified CW damage threshold of an optic, it is necessary to know the following:

1. Wavelength of your laser
2. Beam diameter of your beam (1/e²)
3. Approximate intensity profile of your beam (e.g., Gaussian)
4. Linear power density of your beam (total power divided by 1/e² beam diameter)

Thorlabs expresses LIDT for CW lasers as a linear power density measured in W/cm. In this regime, the LIDT given as a linear power density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size, as demonstrated by the graph to the right. Average linear power density can be calculated using the equation below. 

The calculation above assumes a uniform beam intensity profile. You must now consider hotspots in the beam or other non-uniform intensity profiles and roughly calculate a maximum power density. For reference, a Gaussian beam typically has a maximum power density that is twice that of the uniform beam (see lower right).

Now compare the maximum power density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately. A good rule of thumb is that the damage threshold has a linear relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 10 W/cm at 1310 nm scales to 5 W/cm at 655 nm):

While this rule of thumb provides a general trend, it is not a quantitative analysis of LIDT vs wavelength. In CW applications, for instance, damage scales more strongly with absorption in the coating and substrate, which does not necessarily scale well with wavelength. While the above procedure provides a good rule of thumb for LIDT values, please contact Tech Support if your wavelength is different from the specified LIDT wavelength. If your power density is less than the adjusted LIDT of the optic, then the optic should work for your application. 

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. The damage analysis will be carried out on a similar optic (customer's optic will not be damaged). Testing may result in additional costs or lead times. Contact Tech Support for more information.

Pulsed Lasers

As previously stated, pulsed lasers typically induce a different type of damage to the optic than CW lasers. Pulsed lasers often do not heat the optic enough to damage it; instead, pulsed lasers produce strong electric fields capable of inducing dielectric breakdown in the material. Unfortunately, it can be very difficult to compare the LIDT specification of an optic to your laser. There are multiple regimes in which a pulsed laser can damage an optic and this is based on the laser's pulse length. The highlighted columns in the table below outline the relevant pulse lengths for our specified LIDT values.

Pulses shorter than 10^-9 s cannot be compared to our specified LIDT values with much reliability. In this ultra-short-pulse regime various mechanics, such as multiphoton-avalanche ionization, take over as the predominate damage mechanism [2]. In contrast, pulses between 10^-7 s and 10^-4 s may cause damage to an optic either because of dielectric breakdown or thermal effects. This means that both CW and pulsed damage thresholds must be compared to the laser beam to determine whether the optic is suitable for your application.

When comparing an LIDT specified for a pulsed laser to your laser, it is essential to know the following:

1. Wavelength of your laser
2. Energy density of your beam (total energy divided by 1/e² area)
3. Pulse length of your laser
4. Pulse repetition frequency (prf) of your laser
5. Beam diameter of your laser (1/e² )
6. Approximate intensity profile of your beam (e.g., Gaussian)

The energy density of your beam should be calculated in terms of J/cm². The graph to the right shows why expressing the LIDT as an energy density provides the best metric for short pulse sources. In this regime, the LIDT given as an energy density can be applied to any beam diameter; one does not need to compute an adjusted LIDT to adjust for changes in spot size. This calculation assumes a uniform beam intensity profile. You must now adjust this energy density to account for hotspots or other nonuniform intensity profiles and roughly calculate a maximum energy density. For reference a Gaussian beam typically has a maximum energy density that is twice that of the 1/e² beam.

Now compare the maximum energy density to that which is specified as the LIDT for the optic. If the optic was tested at a wavelength other than your operating wavelength, the damage threshold must be scaled appropriately [3]. A good rule of thumb is that the damage threshold has an inverse square root relationship with wavelength such that as you move to shorter wavelengths, the damage threshold decreases (i.e., a LIDT of 1 J/cm² at 1064 nm scales to 0.7 J/cm² at 532 nm):

You now have a wavelength-adjusted energy density, which you will use in the following step.

Beam diameter is also important to know when comparing damage thresholds. While the LIDT, when expressed in units of J/cm², scales independently of spot size; large beam sizes are more likely to illuminate a larger number of defects which can lead to greater variances in the LIDT [4]. For data presented here, a <1 mm beam size was used to measure the LIDT. For beams sizes greater than 5 mm, the LIDT (J/cm2) will not scale independently of beam diameter due to the larger size beam exposing more defects.

The pulse length must now be compensated for. The longer the pulse duration, the more energy the optic can handle. For pulse widths between 1 - 100 ns, an approximation is as follows:

Use this formula to calculate the Adjusted LIDT for an optic based on your pulse length. If your maximum energy density is less than this adjusted LIDT maximum energy density, then the optic should be suitable for your application. Keep in mind that this calculation is only used for pulses between 10^-9 s and 10^-7 s. For pulses between 10^-7 s and 10^-4 s, the CW LIDT must also be checked before deeming the optic appropriate for your application.

Please note that we have a buffer built in between the specified damage thresholds online and the tests which we have done, which accommodates variation between batches. Upon request, we can provide individual test information and a testing certificate. Contact Tech Support for more information.

[1] R. M. Wood, Optics and Laser Tech. 29, 517 (1998).
[2] Roger M. Wood, Laser-Induced Damage of Optical Materials (Institute of Physics Publishing, Philadelphia, PA, 2003).
[3] C. W. Carr et al., Phys. Rev. Lett. 91, 127402 (2003).
[4] N. Bloembergen, Appl. Opt. 12, 661 (1973).