Determining a Camera’s Low Illumination Performance
How can you determine what type of image a camera will provide in a low light environment? After all, there is really no way that you can get your hands on the actual cameras and test them, one by one, in the field to determine which camera will perform best in a specific low illumination locale.
It’s a major problem for most integrators, who typically are confronted with a wide selection of camera choices from a variety of manufacturers, all with contrary and confusing parameter specifications. For instance, most understand that the higher the megapixel count is, the smaller the pixel size will be. Smaller pixel sizes yield a smaller light gathering surface, which means less sensitivity to light. Nonetheless, it is virtually impossible to determine which camera will perform best by quickly comparing data sheets. So what can integrators, and their customers, do?
Good news! There is a way to measure a camera’s performance in low-light conditions by analyzing the technical specifications. However, it is not a simply crosscheck of one data sheet with another.
Don’t Rely on a Bad Assumption
If the sensitivities of Camera A and Camera B are 0.1 LUX and 0.05 LUX, would you assume that the low illumination performance of Camera B will be better? Many may be tempted to assume this but they would be wrong. If you judge only the sensitivity values, you easily can end up with the wrong camera.
Here’s why: Yes, the parameter of 0.1 LUX means that Camera A can work in a 0.1 LUX illumination environment, but what is its output video in a low-light condition? Perhaps the camera’s sensitivity is 0 LUX when disregarding infrared light imaging. Does this mean the camera can provide images when there is no light? Of course not! The images will be totally dark and useless.
Since both the testing condition and display results must be specified when describing the low illumination value of a camera, try using this formula for both color and black and white modes:
- Color Mode: XXLUX @FX (X IRE, AGC XX)
- B/W Mode: XXLUX @FX(X IRE, AGC XX)
As a result, your new value will be based on at least three parameters, the lens’ F-stop value, the Institute of Radio Engineers (IRE) signal value and the camera’s auto gain control (AGC) status. If these three parameters are provided (for day/night cameras, they are needed for both color and black and white mode), it’s possible to determine which camera will be the best choice in low light.
Let’s Get Into the Details
Let’s start with the F-stop value, the aperture coefficient of a lens. We all understood that an aperture gets larger to allow more light in when the F-stop value decreases and gets smaller when the F-stop value increases. In other words, the F-stop value is in inverse proportion to the amount of light available. Since the amount of light that gets into the camera is decided by the lens aperture, the F-stop value becomes an important variable for low illumination performance. Thus, to test a camera’s low illumination performance, it is necessary to open the aperture to the utmost to ensure that the maximum light gets in. The greater the amount of light, the greater the video signal amplitude will be.
Now, let’s review the IRE value. The lower the IRE value, the darker the black areas are, while the higher the number, the lighter they are. Generally speaking, the IRE value expresses the video signal amplitude. A video signal is useless when lower than 25 IRE (175 mV) since the video displayed on the monitor will be totally dark. Therefore, the IRE value is most often described as a number above 25, generally around 30 and, sometimes, as high as 50.
Thus, the video amplitude and IRE value decrease as illumination is reduced. You want the IRE value to be as small as possible when measuring for the low illumination index.
The last variable to understand is the AGC value. Cameras have an amplifier that magnifies the original video signal. A digital signal processor (DSP) chip detects this signal when sent from the charge coupled device (CCD) sensor. If the original signal is poor, the DSP instructs the amplifier to magnify this signal. This defines the AGC function. When activated, the amplifier automatically magnifies the video amplitude under faint light. However, the amplifier is active only the AGC function turns “on.”
Beside the AGC “on” and “off” functions, some cameras also have other options that adjust magnification, such as AGC 20 dB and 40 dB. These values must also be described when expressing low illumination performance.
With the above terms understood, we can now determine how a camera’s low illumination index is created. For example, if a camera’s low illumination index in black/white mode is 0.24LUX @F1.2 (25 IRE AGC “on”), it means the illumination of the viewed object is 0.24 LUX under the following conditions:
- The AGC function is activated.
- The F value of the lens is 1.2 and the aperture is opened wide.
- The video amplitude is 175 mV.
Now, we can compare the low illumination of two cameras. However, we must be sure that the following conditions are met:
- The color mode or black/white mode is identical.
- The amount of light is identical (i.e. the F-stop value is identical)
- The IRE value is identical.
- The AGC function is identical (“on,” “off,” or “maximum”).
At this point, we can return to the problem discussed at the very beginning. If the low illumination of Camera A is 0.1LUX@F1.2 (30 IRE AGC “on”) and Camera B is 0.05LUX@F1.2 (30IRE AGC “on”), we can clearly say that the low illumination of Camera B is better than Camera A.
Why is this? Camera A needs an environmental illumination of 0.1 LUX and Camera B needs only 0.05 LUX to produce the same signal amplitude in the same conditions. If we place the two cameras in the same environment with 0.1 LUX, we can infer that the amplitude of Camera B will be more than 30 IRE and the video shown in its monitor will be brighter than that of Camera A.
A problem we will encounter looking at cameras from different manufacturers is that the parameters to put in our formula after “@” are seemingly always different. Nonetheless, we can still roughly judge the low illumination performance using the above method.
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