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Characteristic of Oblique Incident Light
When reading a book at night, you will find that the brightness differs
between reading under a lamp or light and reading a little farther from the
light. At that time, you can see that the words are more legible if you turn
the book toward the light. Oblique incident light has the characteristic that
when the angle between the tangential line of an illuminated surface and the
direction of incidence is supposed to be
θ
, the illuminance of the surface is
proportional to cos
θ
. This characteristic is also standardized. If the cosine
law did not hold true, the intensity of illumination from a light source in an
oblique direction could not be measured accurately. Figure 13 shows the
characteristic of oblique incident light.
Relative
illuminance
value (%)
Angle of oblique incidence (°)
Tolerance compatible
with Class A in the
new JIS standard
Figure 13. Characteristic of Oblique Incident Light
