Photon FAQs - Optical

1. How can I figure out which lens version will work best for my application?

There are three variables that need to be known in order to determine the most appropriate lens for an application:

1. The distance from the camera to the object being imaged. This is usually expressed in feet or meters.
2. The size of the object being imaged. This is usually the largest dimension, also in feet or meters, as long as the same units are used.
3. The number of pixels that the object needs to cover in the image, usually using the larger of the horizontal or vertical dimension.

Using these variables, it is possible to calculate the optimal lens, since the sensor resolutions (640x480, 320x240, 160x120) and pixels sizes (25 microns for the Photon 640; 38 microns for other Photon versions) are known values. We provide a calculator for anyone to use at www.corebyindigo.com/tools/LensCalculator/new/ or by clicking the Lens Help button at the Photon home page.

2. Is it possible to get the Photon camera with an athermal lens?

All of the currently advertised Photon lenses are of a passive athermal design. This means that the lenses mechanically self-adjust to changes in temperature such that the lens remains in focus over wide ambient temperature ranges.

3. I need to use the Photon to image an object at close range. Is it possible to adjust the focus of Photon lenses manually? What is the range of focus? What is the close focus distance?

We describe the range of focus to mean the hyperfocal range, that is, the range between infinity and some point less than infinity through which the lens remains in focus. All Photons with lenses are factory-calibrated with the lens locked at infinity focus.

All of the currently available Photon 320 lenses (14.25mm, 19mm, 35mm, 50mm) use a screw-thread mount. The lens focus is locked by a small setscrew. All of the currently available Photon 640 lenses (35mm, 50mm) also use a screw-thread mount, however the Photon 640 lens adapters use a collar with a socket-head cap screw instead of a setscrew.

To adjust the lens focus to something other than infinity focus, or to achieve the shortest possible focus, requires that the setscrew or socket-head cap screw be loosened. It may not be possible to lock the focus at a close focus point since in some cases the lens in almost out of its holder. FLIR does not offer spacers for macro focus. Extreme care should be used to avoid cross-threading the lens in the lens adapter.

The table below shows the approximate minimum focus distance for each Photon 320 lens type. Although Photon lenses are capable of focusing to shorter object distances, it is important to note that they are not designed, calibrated, or specified for this purpose. All Photon camera/lens calibrations are performed at infinity focus. Image space f/numbers can change under extreme finite conjugates. Very close focus applications using a Photon calibrated at infinity focus may result in possible image anomalies, non-uniformities, and/or degraded performance. Customers should be aware of these limitations and evaluate the images for issues.

Lens Focal Length	14.25mm Lens	19mm Lens	35mm Lens	50mm Lens
Close Focus Distance	~5 inches	~12 inches	~27 inches	~72 inches

4. If a lens-less camera core is purchased, how can it be calibrated with a lens?

FFor qualified customers that furnish their own optics for use with Photon cores, FLIR sells a WindowsTM application program called Alt Lens Cal. This software allows users to perform a supplementary calibration of the camera with a lens. This field-calibration process requires the use of at least one blackbody source (a highly uniform, controllable temperature reference) that has an area greater than the diameter of the front of the lens. It also requires a customer-furnished PC, which should be dedicated to this task.

The Alt Lens calibration routine calculates gain terms on a per-pixel basis with the customer-supplied lens attached to the Photon core, and stores the customer-performed calibration in non-volatile camera memory. The original factory calibration coefficients are first uploaded from the camera and stored into a file on the PC, then the new calibration data is downloaded and stored directly into the camera. Multiple calibration files can be stored on the host computer. The original factory calibration file can be restored if necessary, and the user can actually build a library of lens calibration files for a Photon camera.

The current version of ALC software is part number 110-0106-72, and supports all versions of Photon cameras delivered beginning in 2008. Contact FLIR for ALC software for Photon cameras delivered prior to 2008.

5. What are the lens options and the minimum focus lengths for the Photon? What is the range of focus? Is it possible to adjust the focus of Photon lenses manually?

Focal Length	6.3mm	14.25mm	19mm	35mm	50mm
f/number	1.2	1.3	1.4	1.4	1.7
Field of View¹
320x240	-	46° x 36°	36° x 27°	20° x 15°	14° x 11°
160x120	52° x 40°	-	18° x 14°	-	-
640x480	-	-	-	26° x 20°	18° x 14°
IFoV² (milliradians)
320, 160	6.032	2.667	2.000	1.086	0.760
640	-	-	-	0.714	0.500
Minimum Focus Distance	~5 inches	~5 inches	~12 inches	~27 inches	~72 inches
Hyperfocal Distance³	0.25m	5 meters	7.5 meters	26 meters	35 meters
Hyperfocal Depth of Field⁴	-	2.5 meters	3.8 meters	13 meters	18 meters
Weight (Lens & Lens Mount only)	55g	56g	33g	88.5g	128g
Length (Lens only)	0.377"	1.209"	0.769"	1.709"	2.632"
Diameter (maximum)	1.250"	1.654"	1.024"	1.654"	1.772"
Coating Type	High Efficiency	High Durability		Hard Carbon⁵
Nominal Wavelength	8.0 to 14.0 microns

1 Field of View describes the angular measure of a scene imaged with the given pixel array, expressed as degrees in horizontal by vertical directions.
2 Instantaneous Field of View is the angular measure of a single pixel. Small angles are usually measured in milliradians. The IFoV is the pixel size (in microns) divided by the lens focal length
3 Hyperfocal distance is the distance beyond which all objects are acceptably sharp, for a lens focused at infinity.
4 Hyperfocal depth of field is the zone of acceptable sharpness. Increasing the depth of field increases the sharpness of an image. Smaller apertures (higher f/numbers) increase the depth of field.
5 Coating Type for 35mm and 50mm configurations of Photon 640 will be High Durability, not Hard Carbon.

6. Is the spectral response of the microbolometer array 7.5µ to 13.5µ, or is the spectral response a function of the lens?

The spectral response of the microbolometer is influenced by all of the optical interfaces between the focal plane array and the object being imaged. These include the sensor vacuum package front window, the lens, any protective windows in front of the lens, and finally atmospheric transmission between the object and the camera. Standard configuration vacuum packages (as of Jan 2007) have front windows that define the short wavelength cut-on of 7.5 um, while the bolometer itself defines the long wavelength cut-off of 13.5 um, assuming negligible atmospheric attenuation.

7. Should I clean the Photon lens, and how?

A light dusting of air should be enough to dislodge any dust particles, although small amounts of dust will not affect image quality noticeably. The antireflection coating on the germanium optics is easily scratched and the lens should only be cleaned according to the following procedure: If it is absolutely necessary to clean the lens surface, use 75% isopropyl alcohol and lens tissue, and use extremely light wiping motions. Use a fresh section of tissue with each swipe so as not to drag a piece of dirt back over the lens surface.

8. How do water droplets on the camera lens affect the image? Do they affect the image as much as a on a visible light range camera?

Water droplets are entirely opaque in the thermal IR. How they affect the image will depend on several factors. Lens focal length and focus position are two of the important ones after total percent of coverage. If the lens is focused at infinity, rain or water drops will be more out of focus than if the lens is focused up close. A longer focal length lens will also generally cause the droplets to be more out of focus. If the droplets are sufficiently out of focus (they should be if the lens is focused at infinity) then water drops on the lens will reduce the image contrast in proportion to the ratio of obscured area to total lens front surface area.

9. Is there a good reflector for IR? Can a standard front surface mirror be used or are there other materials available?

The best choice for a reflector is a front surface gold coated mirror. The next best choice is aluminum – a common material for front surface mirrors. Other polished metals would also work, but they tend to be slightly less reflective or are prone to oxidation.

10. What are the detection ranges for the various Photon/lens versions?

This question has many variables that must be considered, and some of those variables are inter-related. We make some assumptions to simplify the problem, but it is important to note that this type of question (what can be seen at what distance with an particular IR camera system) is typically approached in terms of target detection, recognition & identification ranges, along with an associated probability of success. Typically the result will be communicated in terms of a percentage probability of a particular target being detected, recognized and identified and certain ranges. There is a standard set of conditions regarding targets and defined meaning for the results that allows some simple test measurements to be fed into models that provide range outputs. Where applicable, our assumptions are consistent with standard range modeling.

We have compiled two charts that provide the nominal detection, recognition, and identification ranges for the different Photon versions. One chart is based on a person-sized object the other on a 2.3 meter vehicle-sized object.

1) Detection, Recognition, & ID of Human Target
2) Detection, Recognition, ID of 2.3m Vehicle

11. Can the Photon camera be pointed at the sun?

We do not recommend intentionally viewing the sun, but looking at the sun will not permanently damage the sensor. It will, however, take some time for the camera to recover. The amount of time needed for recovery will depend on how long the camera was exposed to the sun. The longer the exposure, the longer the recovery time needed.

12. Does the digital zoom feature affect the LVDS output?

For Photon 320 and Photon 640, the zoom feature only affects the analog video.
For Tau, the zoom feature only affects the analog and BT-656 video.
The LVDS data is not affected by the zoom feature in either Photon or Tau cameras.

13. What material is used for the Photon 320 sensor window? Does the window have an anti-reflection (A/R) coating? Can I use a Photon 320 to image below 8µ?

The sensor window used in the Photon 320 camera is silicon, and yes, it has an A/R coating. Without an A/R coating, the responsivity (sensitivity) of the sensor would be significantly degraded, as the transmission of a bare silicon window is about 50% across the IR spectrum. The A/R coating we use has an integral Long Pass (Solar Blocking) filter. This filter suppresses transmission below 7µ. For customers that want to image below 8µ, the coating will be an inhibitor.

The microbolometer structure itself is “tuned” for the LWIR (8µ to 14µ) part of the spectrum, so even with a broadband A/R coating the sensor’s sensitivity would be degraded below 8µ.

Photon FAQs - Optical Questions