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Light intensity or Grayvalue?

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Steven on 23 Jun 2015
Commented: Walter Roberson on 25 Jun 2015
I have a trivial question! I looked about it, but I am still not sure! Something simple confuses me! I would be grateful if someone can clear me!
In a digital image, is the "light intensity" exactly the same as "grayvalue"?
By light intensity I mean the power (energy) of the light source such as laser, and by the grayvalue, I mean the grayscales of the resulting recorded digital image (due to that light)?
I have seen in some book that these two are related, but it did not say how. I saw somewhere else that these two are the same! That is why I asked this question!
Thank you so much!


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Accepted Answer

Jan on 23 Jun 2015
The intensity of the light is a physical value. The gray value of a digitized image depends on the hardware for digitizing, e.g. the sensitivity of the light sensor, the exposure time and the aperture. In addition the processing can influence the gray value, e.g. saturation (when the maximum value is reached) and interpolation, when the output size does not equal the number of sensors.
In the ideal case, a pixel with a higher intensity has a higher gray value. Depending of the hardware of the sensor, there will be a roughly linear relation between the intensity and the gray value - insider certain limits and with a limited precision. So "these two are related" is an accurate description.


Steven on 23 Jun 2015
Thank you Jan for your reply.
Do you know any reference from which I can find this "roughly linear relation?
Thanks again!
Walter Roberson
Walter Roberson on 25 Jun 2015
Some sensor technologies are non-linear over their entire range, and some are non-linear mostly near the limits of their range. At the low end it is not uncommon for there to be a threshold current required to allow a flow with it being relatively easy for additional current to flow after that, so the step between 0 and the first measurable current might be much much larger than the step after that. On the upper end, right near the top, sensors may have nonlinear response in order to avoid burnout or oversaturation in response to conditions that should be expected in the field (e.g., a portable low-light sensor is probably going to get exposed to sunlight at some point.)

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