Miniature Finite Conjugate Objectives and Scan Lenses

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Figure 1.1  The D3X-45P1 miniature objective used with our Mini2P Two-Photon Imaging System.

Features

• Miniature Finite Conjugate Objectives and Scan Lenses
• Interchangeable Objectives for Various Applications:
• Optimized for Imaging Through Prisms, GRIN Lenses, or Thin Windows
• 3X and 7X Magnifications
• Antireflective (AR) Coatings Across the Visible and Near-IR Wavelength Ranges
• Numerical Apertures (NA) Up to 0.70
• Ideal for Two-Photon Microscopy, GCaMP Imaging, and Functional Imaging Applications

Thorlabs offers Miniature Finite Conjugate Objectives and Scan Lenses that are designed for use in miniature two-photon microscopes, such as our Mini2P Two-Photon Imaging System. These compact objectives and scan lenses are AR-coated for lower reflectance at wavelengths in the visible and near-infrared commonly used in two-photon microscopy. 

Each miniature objective is equipped with M5 x 0.5 threading, enabling interchangeability based on the user’s application. Designed for optimal imaging through prisms, GRIN lenses, or thin windows (See Figure 1.3), these objectives support imaging at various tissue depths and orientations. However, their use is not limited to these applications, offering flexibility for a wide range of techniques. The finite conjugate objectives can be used together with miniature scan lenses for two-photon microscopy, GCaMP imaging, and functional imaging applications. Video 1.2 shows the spontaneous signal of a mouse somatosensory cortex expressing GCaMP6 and was taken with the Thorlabs’ Mini2P Microscope and a D3X-45P1 miniature objective. Please see the Specs tab for additional details on the specifications of these miniature objectives and scan lenses, and the Application Info tab for more information on how these values are defined.

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Figure 1.3 The miniature objectives are optimized for imaging through prisms, GRIN lenses, and thin windows. Figure reproduced from Large-Scale Two-Photon Calcium Imaging in Freely Moving Mice.¹

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Figure 1.4  A D3X-45P1 miniature objective and an LSM5-M2P scan lens.

References

1. Zong, W. et al., "Large-scale two-photon calcium imaging in freely moving mice", Cell Vol. 185, Issue 7, 1240-1256 (2022)

1. Zong, W. et al., Large-scale two-photon calcium imaging in freely moving mice, Cell Vol. 185, Issue 7, 1240-1256 (2022)

Scan lenses are utilized in a variety of laser imaging systems, including confocal laser scanning microscopy and multiphoton imaging systems. In these applications, a laser is precisely scanned across the sample to provide point-by-point illumination, enabling the reconstruction of high-resolution 2D and 3D images. One of the functions of a scan lens in a laser scanning microscope is to project the scanning mirror to the entrance pupil of the objective such that it effectively scans at the objective entrance pupil. The system described below utilizes a miniature scan lens and a finite conjugated miniature objective. The scanning mirror, defined as the entrance pupil of the scan lens in Figure 3.1, is located to the left of the scan lens. The scan lens projects the laser input beam originating from the entrance pupil and focuses the rays on an intermediary plane to the right, defined as the image plane. The separation between the scan lens and the objective should be set such that the image plane of the scan lens coincides with that of the objective. The beam projected from the scanning mirror onto the image plane acts as the laser input for the objective, and the objective focuses this beam onto the sample plane to the right. The distance from the front element of the objective to the sample plane is defined as the working distance of the objective.

If the beams are extended from the image plane, they converge on a plane to the right—designated as the exit pupil of the scan lens and labeled in Figure 3.1 as the Virtual Aperture. Similarly, for the objective, its entrance pupil is virtually located outside the physical lens body and is also labeled as the Virtual Aperture. The position of the virtual aperture relative to the image plane is defined as the entrance pupil position of the objective, and the diameter of the virtual aperture is defined as the entrance pupil diameter of the objective (labelled as the Entrance Pupil Diameter in Figure 3.1). It is important to note that the objective's entrance pupil position and diameter can be much larger than the physical size of the objective. A laser scanning system design should ensure that the exit pupil position of the scan lens matches the entrance pupil position of the objective to achieve an optical field of view (FOV) with uniform illumination. For this to be possible, the scanning mirror must be offset from the front focal plane of the scan lens, causing the beams extending from the image plane to converge at a finite virtual position. 

Figure 3.1 Visual Definitions of the Terms Described in Table 3.2 (Not to Scale)