DeltaE Lab

A color difference calculator

DeltaE Lab is an app for Apple iPhone that calculates the difference between two colors (respectively the reference color and the sample color) according to some standard formulas. For general information about color difference, see WIkipedia, ColorWiki and Bruce Lindbloom website.

DeltaE Lab works on iPhone, iPod Touch and iPad with OS 4.2 or better. As well as English is localized in Italian and German.

On startup, the app displays a number of empty text fields (see first screenshot below). To become familiar with the calculator, tap the button on the top right corner. Each time you tap this button the app loads two random colors (see second and third screenshots below). A double tap clears all fields.

You enter data of both colors (reference color and sample color) in the fields with white background. The results appear in the fields with colored (yellow and green) background.

The data entry area is divided into two columns: the first (reference) color data should be placed in the first column and the second (sample) color data in the second column. Initially all fields are empty, which means that all values are undetermined. Note that an empty field does not indicate 0; it indicates instead that the value is undetermined.

To enter values, tap a field with white background to display the keyboard. You can keep the keyboard open until all data have been entered.

Typically you start by entering the reference color's L* value. This is a float number from 1 to 100.

Next, enter the a* and b* pair. C* and h values will be immediately calculated and will automatically populate the relevant fields. Alternatively you can enter the C* and h values; in this case the a* and b* values will be calculated and the relevant fields will be automatically filled in. Data can enter in the fields in any order.

By definition, you can enter the following numbers in the fields with white background:

• L* (lightness) must be a number between 0 and 100;
• a* and b* can be any number, positive or negative, and also 0;
• C* (chroma) cannot be negative, so it must be 0 or a positive number;
• h* (hue angle) is an angle and therefore must be a value between 0° and 360°.

Please note that a* and b* values are independent, in the sense that a* can be any value, positive or negative, and similarly for b*. However the values C* and h are not independent. Specifically:

• if C* (chroma) is 0, then h (hue angle) is undefined (this is a gray, i.e. an achromatic color);
• if C* > 0, then h must have a definite value between 0° and 360° (this is a chromatic color).

In any case the application tries to intercept any user error and provide a placeholder that appears in each field when the field is empty.

The following controls are implemented:

• In the L* field values less than 0 or greater than 100 are rejected: if you type a value greater than 100 it is replaced with 100;
• In the a* and b* fields, any value–positive or negative, or zero–is accepted;
• In the C* field values less than 0 are rejected;
• In the h* field values less than 0° or greater than 360° are rejected. (The degree symbol is inserted automatically, you don't have to enter it.) A value exceeding 360° is replaced with an equivalent angular value between 0° and 360° (for example 450° is replaced with 90°, because 90° is equivalent to 450° and is within the accepted range).

Having entered the reference color (first screenshot above), insert the sample color in the same way. As you enter the values, the application calculates the individual differences and enters them in the third column (field with yellow background, second screenshot above). When all color values have been entered, the first 15 fields are filled (third screenshot above).

Now you can tap the Fatto button, and the calculator reveals the various results (fourth screenshot above).

The calculator computes following individual differences:

• simple differences ΔL* (lightness difference), Δa*, Δb*, ΔC*, Δh (hue angle difference);
• the hue difference ΔH*.

Next, the application calculates various color differences ΔE*. As you know, color differences can be calculated according to different formulas. The application uses the six following formulas:

• ΔE* according to CIE 1976: this is the original value, still used in some field, for example in the ISO standards for offset printing;
• ΔE* according to CIE 2000: this is the latest standard proposed by CIE, used for example to measure monitor colors;
• ΔE* according to CIE 1994: an intermediate standard with two variants that correspond to two different formulas:
  • Graphic arts;
  • Textiles;
• ΔE* according to CMC 2:1: a formula with two variants:
  • Acceptability;
  • Perceptability.

The number of decimal places for all fields can be changed by using the slider at the bottom. You can set a value between 0 and 4, before or after making the calculation.

You can also swap colors, by dragging a color swatch over another. In this case the reference color become the sample color and vice versa. Swapping colors can be used to verify commutativity of the ΔE* formulas (left screenshot below).

Since many colors are out of the screen gamut, the color of the two swatches are only approximate. For the in-gamut colors, it is assumed that the screen is sRGB and Lab D65.

When you tap the information key (circle "i" icon in bottom right corner), the information screen is displayed (right screenshot above). The link at the bottom of this screen allows you to load the web page you are reading. To return to the main screen, tap the Done button, in the upper leftcorner.

DeltaE Lab is a project by Mauro Boscarol. For information and requests, send an email to me.