Xjyuk

I have industrial camera. Its model is BFLY-23S6M as depicted below. This camera is used for license plate recognition. A 850nm infra-red projector is near the camera. As I know it is better to reject visible light.

There is a mirror-like object after the sensor. You can see it in silver color. I want to know what it is, and what its usage is. Is it a filter (eg, anti-glare or band pass)?

From the documentation, I know the transparent filter (for monochrome) and non-transparent filter used for color cameras. This mirror-like one is a custom user installed and is not a default just for dust protection.

From the model number (BFLY-23S6M) it would seem that your camera is a monochrome Blackfly GigE. This camera has two pieces of glass between the lens and sensor image plane - in the monochrome model, these are the sensor package window and a transparent piece of glass which protects the sensor from dust.

4.4.1 Back Flange Distance
The Back Flange Distance (BFD) is offset due to the presence of both a 1 mm infrared cutoff (IRC) filter and a 0.5 mm sensor package window. These two pieces of glass fit between the lens and the sensor image plane. The IRC filter is installed on color cameras. In monochrome cameras, it is a transparent piece of glass. The sensor package window is installed by the sensor manufacturer. Both components cause refraction, which requires some offset in flange back distance to correct. The resulting C-mount BFD is 17.99 mm. The resulting CS-mount BFD is 12.52 mm. For more information about the IRC filter, see Infrared Cut-Off Filters.

4.6 Dust Protection
The camera housing is designed to prevent dust from falling directly onto the sensor's protective glass surface. This is achieved by placing a piece of clear glass (monochrome camera models) or an IR cut-off filter (color models) that sits above the surface of the sensor's glass. A removable plastic retainer keeps this glass/filter system in place. By increasing the distance between the imaging surface and the location of the potential dust particles, the likelihood of interference from the dust (assuming non-collimated light) and the possibility of damage to the sensor during cleaning is reduced.

4.7 Infrared Cut-Off Filters
FLIR machine vision color camera models are equipped with an additional infrared (IR) cut-off filter. This filter can reduce sensitivity in the near infrared spectrum and help prevent smearing. [...] In monochrome models, the IR filter is replaced with a transparent piece of glass.

_{Source: Pages 37-40 of the PGE Technical Reference manual which can be found at FLIR support centre.}

Although the plastic retainer is described as 'removable' in section 4.6, the manual also carries the following warnings:

• Cameras are sealed when they are shipped. To avoid contamination, seals should not be broken until cameras are ready for assembly at customer's site.


• Use caution when removing the protective glass or filter. Damage to any component of the optical path voids the Hardware Warranty.


• Removing the protective glass or filter alters the optical path of the camera, and may result in problems obtaining proper focus with your lens.

Maybe the camera has been equipped as an infrared camera? The infrared filter should then reflect visible light since absorbing it would result in a heat source right before the sensor which would interfere with infrared imaging.