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Colorimeter

A colour measuring device for measuring/comparing colours.
It is often used for the calibration of monitors or in biology/chemistry for analysis. More powerful devices, such as the spectrophotometer, can measure colours spectrally and are also used, among other things, for proofs to verify or calibrate printers.

Colour distance

The measured distance (colour difference) between two colours. The colour locations of the two colour values are displayed within a system and the difference is given in Delta-E.

Colour measurement

Colour measurement is used to determine the exact colour location of a colour in a colour system.

There are three methods:

The Tristimulus Method:
A tristimulus colourimeter shines an internal light source onto the surface of the colour sample.The reflected light passes through three filters – red, green and blue – which determine the intensity of these color components.
The reflected light passes through three filters – red, green and blue – which determine the intensity of these colour components.

The Spectral Method:
A spectrophotometer works in a very similar way – but has not only three, but 31 filters that measure 31 different wavelengths of the reflected light. Thus the spectral method is one of the best methods for measuring colour.

Equality Method:
Here, a measuring device (or the eye) is used to compare the colour sample with standard colour samples until all samples are considered identical by the observer. However, this is assessed subjectively by the viewer, which is why the other methods of colour measurement are preferred in practice.

Delta-E

Delta-E is a unit for the colour distance between two colours. The larger the number, the greater the “distance” between the colours. The main idea is that a Delta-E colour distance – in any direction – looks the same to the human eye. So if one colour is brighter by 5 Delta-E than another, to the human eye it looks just as distant from a colour that is redder or bluer by 5 Delta-E.

The most common method is Delta-E76 or CIE 1976 / CIE76.

This method uses the Euclidean distance, between the LAB colour values of the colour.

The calculation formula is: Square root of (L-distance squared) + (a-distance squared) + (b-distance squared)

Example: How far apart are the colours in Delta-E76: 
Colour 1: LAB 90/65/55
Colour 2: LAB 87/65/58

Calculation:
square root of [((90-87) = 3 squared) + ((65-65) = 0 squared) + ((55-58) =-3 squared)]
=
Square root of [(9) + (0) + (9)]
=
Square root of (18)
= 4,24

So CIE76 is quite easy to calculate, but the Delta-E76 calculation has its drawbacks, as it has been shown that the same Delta-E76 colour distance from the eye is not really seen as equally spaced, so newer calculation methods were established.

Other common methods are…

Delta-E CMC
Delta-E94 (1994)
Delta-E00 (2000)

Delta-E00 is currently the most common method used in proofing and printing technology, Delta-E CMC is widely used in textile printing.

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