Coulor temperature sensor/meter?

temperature sensormeter

Hello, I was wondering if anyone knows of a circuit designed to measure coulor temperature.
I came across this in my search:

In general, color temperature meters available on the market today are comprised of three silicon photodiodes filtered for red, green, and blue sensitivities appropriate for color film. These devices measure the relative amount of energy present in these three wavelength regions and determine the color temperature based on the shape of the black-body curve that best fits through these metered readings.

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As well as a schematic for measuring all sorts of things light-related (a colorimeter) which used two TCS230 chips.

Basically, my questions are:
Would three photodiodes be accurate for measuring temperature?
Would I be better off with something like the TCS230?
And does anyone know of a circuit that already does this that I could look at?

Thanks,
Spork

temperature sensor meter

Would three photodiodes be accurate for measuring temperature?

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1. Photodiodes without suitable filters can't do anything
2. Temperature is different from color temperature
3. TSC230 needs an additional IR blocking filter for correct operation

colour temperature sensor

First off, I apologize if this is in the wrong section, I could not find one in which this question accurately fit.

I do appreciate your response, but I fear one of two things when reading it:
Either you do not understand the subject and were simply responding just to respond.
or
You are trying to 'dumb down' your responses for me, which I appreciate, but does not help me with my questions.


I would also like to either elaborate or correct information in your response.
First, photodiodes are QUITE useful without filters as they give us the ability to detect light as well as the ability measure the intensity. We just can not tell the color of the light.
Second, you are only partially right about color temperature. It is most definitely an actual temperature, the difference here is when I speak of "measuring color temperature" I am using the common terms to ask about comparing chromaticity of light to that of a black body radiator. The "temperature" is quite literally the hottness (temperature) required to make a black body radiator produce the color in question.

Why do you say that I need an IR filter? Infrared light is about the 1200k-1500k range, without a filter I could measure that as well, right? I don't see why the device should discriminate. In the sense of cinematography, I could see how this would change things (since cameras use IR filters), but in my application I want a true reading of color temperature.

So my questions remain, with some additions:
Would three photodiodes be accurate for measuring (color) temperature?
Would I be better off with something like the TCS230 (than the photodiodes, due to programability/cost/64 sensors as opposed to three)?

When designing a circuit with said photosensors, am I correct to assume that:
The values I read out will be Red,Green, and White presence (respective to each diode) or not with a higher or lower value indicating brightness (luminance) of each. As well as clear/white which gives me an overall value.
Then I take the (r,g) chromaticity, which is represented by:
r=(R)/(R+G+B) (or White/unfiltered diode in this case)
and
g=(G)/(R+G+B) (or White/unfiltered diode in this case)
Then finally, use the chromaticity point in (lets say McCamy's) formula to determine color temperature?
Lastly, is there a known example circuit I could look at?

tsc230 +datasheet

I do appreciate your response, but I fear one of two things when reading it:
Either you do not understand the subject and were simply responding just to respond.
or
You are trying to 'dumb down' your responses for me, which I appreciate, but does not help me with my questions.

Click to expand...

If you review your original question, you'll see that it's very unclear and raises doubts, if you understand what you are doing. Thus I focussed on some basic points involved with your question. I wouldn't say "dumb down", cause I was trying to give some serious information anyway.

In detail:
1+2.

Would three photodiodes be accurate for measuring temperature?

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I simply wondered, if you know, that you have to install three suitable filters with the photodiodes. It's still not explicitely said in your verbose reply, but I expect now, that you know about, also, that you don't measure a temperature.

3. Yes, you need an IR blocking filter with TSC230. If you read the datasheet, you know why. It may also be necessary when using color filters, that have poor IR blocking, with inividual photodiodes. Otherwise you get considerable errors in color temperature measurement depending on IR spectral components.

Although color temperature definition is based on equivalent black body temperature of light sources, the measurement must be restricted to visible spectral components. Unfortunately, sources with similar color temperature can have a very different IR spectrum, consider e. g. a white LED, sunlight and a halogen lamp. (Another aspect of the difference between color and real temperature)

Finally, I think that a TSC230 is cheaper (considering filter costs) and possibly better performing than a set of individual photodiodes, e.g. cause the TSC230 color sensors have a small common aperture while individual diodes may need additional optics to achieve uniform illumination from the source under test.

tcs230 +circuit

Thanks for the response again FvM, quite helpful.

I suppose my original post should have been more along the lines of:
I am planning on using photodiodes and what I plan to accomplish is measuring coulor temperature. Would these be my best option?
I have pretty limited experience with optoelectronics, so I am basing my ideas off what I have seen before, which I know may not be the best idea.



I had overlooked how greatly the readings were affected by IR wavelengths the first time I read it, but I do see the graphs now and it makes sense. Also, yes I understood from the start that each diode would have to be measuring a separate color, which is why I mentioned the color light-to-frequency converter (with the built in filters, except IR apparently).

So the only question that I really have left would be:

Am I going about this the correct way? (Using a TSC230, calculating an r,g coordinate and using a CCT equation to calculate the coulor temperature.)

best color temperature meter 2009

Yes, that seems to a be correct way. I must admit, that I didn't yet use the TSC230, I know it, cause I've been working with other TAOS devices as linear photodiode arrays in optical application. I also know from other visible and IR applications, that the inherent advantages from TSC230 construction details shouldn't be underestimated.

I didn't yet use the CCT calculations, but your approach sounds reasonable. At best, you should have an opportunity to compare with a calibrated color temperature instrument, they are used e.g. in professional graphics design for computer monitor and reference light source calibration.

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