Lighting requirements and calculations for high-speed imaging. A comparison of sensor ISO rating and how it drives the need for more (or less) light

One of the most common questions a photographer will ask is “how much light is needed for my high speed video application?” Some basic and approximate calculations can be made to address this issue. The following information will be required in order to calculate this:

1. The ISO rating of the camera sensor2. The type of lens to be used and its the desired F-Stop3. The frame rate for this given recording4. The electronic shutter speed

The objective is to evenly illuminate the subject with “flicker-free” lighting of a suitable Lux level and Color temperature (Deg K).

The calculation

Lux = (360 X F-Stop^2 X FPS)/ISO

ISO = (360 X F-Stop^2 X FPS)/Lux

We are solving for Lux in this case

Therefore, if you have:• F-Stop = f4• FPS = 1000 • Electronic Shutter Speed = 1/5000) (note use the higher value of the 2- in this

case, frame rate of 1,000 FPS with a 1/5000 shutter we will use 5,000)• ISO = 5000 (Example of NAC GX1 Color camera is being used -which has an

ISO of 5000 color and 20,000 Monochrome)

Lux = (360 X 4 X 4 X 5000)/5000

Lux = (360 X 16 X 5000)/5000

Lux @ f4= (28,800,000)/5000 = 5,760

[Lux @ f5.6 = ~11,300]

In addition to this calculation take the resulting value and multiply by 3.0 “just to be safe” as conditions may call for less efficient lenses, higher frame rates, and higher shutter speeds in the future.

In this example f5.6 can be considered very a desirable F-stop setting and would drive the lighting system design to output 33,900 Lux (11,300 x 3) rounding up I would aim for 35,000 Lux for use with the NAC GX1

The same calculations applied to an ISO of 2,000 (IDT) would call for a lighting system to be designed at 84,672 rounded up 85,000 Lux

The same calculations applied to an ISO of 1,600 (Olympus and Photron) would call for a lighting system to be designed at 105,840. 106,000 Lux

The same calculations applied to an ISO of 1,200 (Phantom) would call for a lighting system to be designed at 141,000 Lux

Summary as it relates to ISO vs required lighting:

NAC estimated requirement = 35,000 Lux

IDT estimated requirement = 85,000 Lux

Photron and Olympus estimated requirement = 106,000 Lux

Phantom estimated requirement = 141,000 Lux

Note: Another way to view this important performance characteristic would be to consider having a 35,000 Lux lighting system and capturing 1000 FPS with a shutter of 1/5000; the following adjustments would then have to be made to each of the cameras:

NAC @ 1/ 5000 shutter Lens = f5.6 camera gain on low

IDT @ 1/5000 shutter Lens = f2.8 camera gain on medium to high

Photron and Olympus @ 1/5000 shutter Lens = f2.8 camera gain on medium or high

Phantom @ 1/5000 shutter Lens = f1.4 (fully open lens) camera gain on high

Why is F-Stop and light sensitivity (ISO Rating) so critical?

As ISO rating decreases the lens must be “opened up”. The F-Stop value on the lens is decreased the lens aperture “opens up” to introduce more light into the image. As the lens “opens up” a number of negative trade-offs are introduced into the image:

1. A loss of image Depth of Field (focus from the front plane of the AOI to Rear plane of the AOI) which is very undesirable.

2. Image distortion or fish-eye effect

3. Need for additional camera sensor gain which amplifies existing visual “noise” in the image.

4. These negative trade offs will also cause inaccuracy in image analysis which could yield poor test results and cause uncertainty as it relates to product engineering decisions.

Depth of field in the image below is quite good. 1/5000 shutter, f5.6 lens, lights measured at 30,000 lux. Notice the clarity in this image. The seat stitching in the foreground is visible and image remains in-focus all the way through to the outside of the vehicle in the background . Further this image has little to no distortion or noise in the image. Why? Light sensitivity and high quality sensor performance. As Sensor ISO ratings fall so does the ability to collect clear accurate images.

The Lighting vs Camera equation:

The customers analysis needs to address whether they want to “fight” camera performance and light sensitivity by simply buying a larger lighting system, boosting the settings on the camera, and opening the lens up to undesirable settings. Alternately they could invest slightly more into cameras that perform better and spend significantly less on the lighting system and its related infrastructure. In the end the overall budget would likely improve by choosing the better camera technology. The overall resulting image quality and system reliability will also be much better.

A final cost analysis will need to be done to validate this conclusion, but the savings on the lighting system could likely save the customer money affording them to buy additional cameras initially. This would also mitigate the frustration that comes from a lower performing camera system. Finally the image quality being sent out into the global crash safety community would be regarded as very high quality images.