Illumination
Main terms and definitions:
- pivot point — a point which is located on the surface of the object (document) and identified as a center when describing its characteristics;
- axis of illumination — a line connecting the light source and the pivot point of the object;
- axis of observation — a line connecting the optical system of the observer’s eyes and the pivot point of the object;
- angle of illumination — an angle between the axis of illumination and the object surface;
- angle of observation — an angle between the axis of observation and the object surface.
Types of light classified by the wavelength:
- UV light — UVA (320–400 nm), UVB (290–320 nm), UVC (200–290 nm);
- white light — visible light with the wavelength of 380–780 nm;
- IR light — IR (740–1500 nm).
Illumination direction (azimuth):
- front illumination is directed from the observer’s side towards the X-axis;
- back illumination is directed from behind the object towards the X-axis;
- side illumination is directed from the right or left sides of the object towards the Y-axis.
Types of light classified by the angle of incidence:
- incident (60-90°);
- oblique (30–60°);
- sliding light (0–30⁰) is used for visualization of surface relief details;
- bottom light is used for image visualization in transmitted light.
Illumination classified by the type of the light flow:
- direct light is a harsh light that creates visible shadows on the object as well as glares on the surface;
- diffused light is a soft indirect light that casts soft shadows and hides relief details of objects.
Fig. 1. Scheme of illumination and observation: I — axis of illumination; O — axis of observation; α — angle of observation; β — angle of illumination
Fig. 2. Illumination direction: a — back illumination; b, c — side illumination; d — front illumination
Fig. 3. Types of light: a — incident; b — oblique; c — sliding; d — bottom