Color temperature meter.
If someone finds the table too inaccurate and would like to use a very impressive-looking light color temperature meter, he would have to shake his pocket very carefully. How can we measure color temperature?? Complicated scientific instruments will be omitted. We will proceed from this in our considerations, that with the increase of the color temperature in the spectral composition of light, the content of the blue and red parts of the spectrum shifts in favor of the former, so the light will be bluer. In the simplest cases, we can use a normal light meter for this purpose, taking two measurements using two filters - blue and red. If the obtained galvanometer deflection ratios are calibrated and a corresponding table is prepared, we can measure the color temperature of light in this uncomplicated way. The Arnz filter factory in Jena produces a blue-red double filter for this purpose, which can be found commercially under the name "Colortester". It can be used with any light meter, however, it is specially adapted to the "Werralux" and "Weimarlux" light meters. It is more elegant to measure using special instruments. They usually consist of two photocells or differential cells, which are covered by filters. The galvanometer is in a bridge arrangement, so that if both cells are equally excited, no current flows. By partially covering them, we can regulate the flow of light to the cells, so that the instrument pointer points to zero. With this adjustment, the scale moves, on which values are given in degrees Kelvin or Mired, or the required filter values in decamireds. The "Sixticolor" light color temperature meter from Gossen works on this principle; The "Lifacolorlux" of Augsburg has a device for automatic diaphragm adjustment.
We can also mention the small accessory "Color-Finder", which works on a completely different principle. It only consists of a small array, on which two red color scales lie next to each other. One of them is made of normal dyes with different color shades, while the other is made of a special dye, whose appearance changes depending on the color temperature of the light falling on it. By comparing the two scales, we can thus determine, which field of the differentiated scale corresponds to the changed color and read the value of the color temperature of the light next to this field.
Also pay attention to this, that the photos are reversible, which will still have color contamination (possibly arising during development), they can be corrected only to a small extent, by chemical means or by wrapping them in corrective foils. We will talk about the further use of filters later.