The Gardner Delta Color Comparator uses precision-polished optical-glass filters as reference standards.
Two different models are available: with or without illumination.
The comparator is an arrangement of two wheels in which nine color filters are imbedded in each wheel. The glass filters range in color from water white through deep amber. A tube of the sample liquid is placed between the two filter wheels. The user then rotates the color wheels until the filter glass closest in color to the liquid is in place. That filter notation then becomes the color description for the liquid.