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The amount of reddening is characterized by color excess, defined as the difference between the observed color index and the normal color index (or intrinsic color index), the hypothetical true color index of the star, unaffected by extinction. For example, in the UBV photometric system we can write it for the B−V color:
The system is defined using a set of color optical filters in combination with an RMA 1P21 photomultiplier tube. [2] The choice of colors on the blue end of the spectrum was assisted by the bias that photographic film has for those colors. It was introduced in the 1950s by American astronomers Harold Lester Johnson and William Wilson Morgan.
A color–color diagram is a means of comparing the colors of an astronomical object at different wavelengths. Astronomers typically observe at narrow bands around certain wavelengths, and objects observed will have different brightnesses in each band.
A commonly adopted standardized photometric system is the Johnson-Morgan or UBV photometric system (1953). At present, there are more than 200 photometric systems. At present, there are more than 200 photometric systems.
The UBV catalog data format is in plain text. After uncompressing by unzip or gunzip it can be seen by any text viewers or by the online ADC viewer. Specifications for the catalog format are provided at the download sites. Original catalogue data, 1953-1985, 109293 objects (alternative reference is Mermilliod, 1987)
For this purpose the UBV system is widely used, in which the magnitude is measured in three different wavelength bands: U (centred at about 350 nm, in the near ultraviolet), B (about 435 nm, in the blue region) and V (about 555 nm, in the middle of the human visual range in daylight). The V band was chosen for spectral purposes and gives ...
As most definitions of color difference are distances within a color space, the standard means of determining distances is the Euclidean distance.If one presently has an RGB (red, green, blue) tuple and wishes to find the color difference, computationally one of the easiest is to consider R, G, B linear dimensions defining the color space.
The shortest wavelength's index is , and the longest's is . Abbe numbers are used to classify glass and other optical materials in terms of their chromaticity . For example, the higher dispersion flint glasses have relatively small Abbe numbers V < 55 {\displaystyle V<55} whereas the lower dispersion crown glasses have larger Abbe numbers.