Photometric and Radiometric measurement concepts

[Aditya Mittal]

Hello,
I am trying to understand the concepts of photometric and radiometric measurements in an electronic sensor. Various photo sensors based on CCD (Charged Coupled Devices) or CMOS photo diode array technology are available in the market for measurements in Visible Spectrum. Now I want to know if there are any generic formulae or functions available to convert the sensor output (analog/digital) into relevant photometric or radiometric quantity such as wavelength or irradiance or Illuminance etc.

 

[NTW]

Without knowing the spectral response of those devices to the whole radiometric interval that you want to measure, I don't think that's possible...

 

[Aditya Mittal]

You mean to say the graphical representation between Wavelength (nm) - x-axis and Responsivity (A/W) - y axis. This information is normally given in data sheet for a particular sensor. So how do I use that information to get the parameters that I need? I have also attached the sample spectral response curve for a typical sensor.

 

[NTW]

If I understand it correctly, that sensor detects in eight bands from 400 to 700 nm. Between those extreme wavelengths there are gaps with no detection. And, for wavelengths longer than 700 nm, the sensor simply doesn't work at all. The black line in the graphic is probably the transmittance of the sensor's window...

 

[Drakkith]

If I understand it correctly, that sensor detects in eight bands from 400 to 700 nm. Between those extreme wavelengths there are gaps with no detection. And, for wavelengths longer than 700 nm, the sensor simply doesn't work at all. The black line in the graphic is probably the transmittance of the sensor's window...

I don't think so. I think the black curve is the sensor's spectral response. The colored lines look like specific filters that are used when you want to view only a specific part of the spectrum.

 

[Drakkith]

Now I want to know if there are any generic formulae or functions available to convert the sensor output (analog/digital) into relevant photometric or radiometric quantity such as wavelength or irradiance or Illuminance etc.

There are. I'll try to find my book on astronomical image processing and see what it says about this. I'm willing to bet it has it in there.

 

[Andy Resnick]

You mean to say the graphical representation between Wavelength (nm) - x-axis and Responsivity (A/W) - y axis. This information is normally given in data sheet for a particular sensor. So how do I use that information to get the parameters that I need? I have also attached the sample spectral response curve for a typical sensor.

Calculating the radiometric quantities (W received, etc) is straightforward from the graph- You start by specifying the (spectral) input radiation, multiply by the (spectral) detector sensitivity and you're basically done.

Calculating the photometric quantities proceeds similarly, but you also multiply the eye's spectral response curve (http://en.wikipedia.org/wiki/Photometry_(optics)#mediaviewer/File:Luminosity.png) to the above calculation to convert watts into lumens.

Is that what you want to do?

 

[Aditya Mittal]

@Andy Resnick - Yes that is true that I want to measure radiometric and photometric quantities from the sensor output. The output of the sensor is preferentially in Volts or Ampere and at most it could be an input to an ADC to get the digital output which can be read by a micro controller section in the circuitry. Now I am still unable to utilize that sensor output.
@Drakkith - It would really be of great help to me if there are any such formulae available in any of the text references that you might have.
@NTW - The response posted by '@Drakkith' is correct to your comment.

In general I want to design a light meter for photopic and scotopic illuminance, EVE factor, luminous color, color rendering index, luminous spectrum. For that I need to first focus on sensor interface and how the output from sensor could be utilized for this.
Also let me know in case you would like to have any other inputs from my end regarding this.

 

[Andy Resnick]

If you don't have access to the spectral content of your illumination and all you have is the voltage/current output of the sensor, there is no way to perform your calculation.