Samples generally can be divided into three categories

(a) non-transparent (opaque reflective) samples
(b) transparent
(c) translucent.

(a) Non transparent (opaque reflective) samples
Some light is reflected and some is absorbed but no light passes through such samples.

(b) Transparent Samples
These samples allow light to pass through essentially unchanged. However, at every boundary between two different materials such as an air-plastic interface, light changes its speed and as a result a small fraction of the light is reflected.

(c) Translucent Samples
In addition to being transmitted and reflected light may also be absorbed. In translucent samples some of the light is absorbed, some passes through the sample unscattered (transmitted) and some is scattered.

When comparing samples visually, e.g., standard to colour match, the samples should have at least one straight edge. This is essential as colours can only be accurately assessed by laying the standard and the reference side by side. It is also important to eliminate the mirror gloss effect for samples with a smooth gloss surface. This can be done by holding the samples at a 45° angle to the incident light. A small stand angled at 45° is useful for this purpose.

Samples should also have the same surface finish. It is impossible to assess accurately colour if one sample has a smooth gloss surface and the other has a textured surface.

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