Routines (alphabetical) > Routines: C > CT_LUMINANCE

# CT_LUMINANCE

The CT_LUMINANCE function calculates the luminance of colors.

This routine is written in the IDL language. Its source code can be found in the file `ct_luminance.pro` in the `lib` subdirectory of the IDL distribution.

## Syntax

Result = CT_LUMINANCE( [R, G, B] [, BRIGHT=variable] [, DARK=variable] [, /READ_TABLES] )

## Return Value

The function returns an array containing the luminance values of the specified colors. If the R, G, and B parameters are not specified, or if R is of integer, byte or long type, the result is a longword array with the same number of elements as the input arguments. Otherwise, the result is a floating-point array with the same number of elements as the input arguments.

## Arguments

### R

An array representing the red color table. If omitted, the color values from either the COLORS common block, or the current color table are used.

### G

An array representing the green color table. This parameter is optional.

### B

An array representing the blue color table. This parameter is optional.

## Keywords

### BRIGHT

Set this keyword to a named variable in which the array index of the brightest color is returned.

### DARK

Set this keyword to a named variable in which the array index of the darkest color is returned.

### READ_TABLES

Set this keyword, and don’t specify the R, G, and B arguments, to read colors directly from the current colortable (using TVLCT, /GET) instead of using the COLORS common block.

## Version History

Pre 4.0 | Introduced

## See Also

GAMMA_CT, STRETCH