DIY Color Temperature Meter
by viilaamo.com
Color Theory
Color temperature can be measured just by comparing the amount of red and blue color in the light. In theory the amount of red and blue as per color temperature is roughly as in the picture below. In practise it is not so beautiful.
The two LDRs are covered with red and blue pieces of glass; glass works as a 'color filter'. We measure the difference of the color balance by using Arduino board analog input pins. Since the glass pieces also 'eat' part of the light then we also measure roughly the lightness in order to compensate the lost light.
Typically the resistance of LDR is mega ohms in dark and tens of ohms in bright light; hundreds when covered with colored glass. Arduino can measure voltage between zero and five volts with 10 bit accuracy. We decided to focus on measuring the most common temperatures (2700K - 6500K), which can be used e.g. for setting the color temperature in DSLR camera. See the HW schematics below.
