Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

TABLE 14-12. Reflecting Materials
NONIONIZING RADIATION SAFETY 747
TYPE MATERIAL REFLECTION COEFFICIENT
Specular Mirrored glass 0.8–0.9
Aluminum foil 0.84–0.87
Rhodium 0.7–0.9
Aluminum, polished 0.6–0.7
Chromium 0.60–0.65
Stainless steel 0.55–0.65
Black structural glass 0.04–0.05
Mixed Aluminum, oxidized 0.70–0.85
Aluminum, brushed 0.54–0.58
Aluminum paint 0.6–0.7
Stainless steel, satin 0.51–0.56
Diffuse White plaster 0.90–0.92
White paint, flat 0.75–0.90
Limestone 0.35–0.65
Sandstone 0.30–0.42
For higher-power lasers, the “safe” viewing distance can become very great. If the
power of the laser in this example is increased to 100 mW, then the “safe” viewing
distance is found to be 3.57 × 10 4 cm, or 357 meters!
Reflections
When a laser beam falls on a surface, some fraction of the incident light, depending
on the nature of the surface and on the wavelength of the light, will be reflected. If the
reflecting surface is polished and mirrorlike and the angle of incidence of the beam
is equal to the angle of reflection, we have specular reflection. On the other hand,
if the reflecting surface is rough or dull and the illuminated surface appears equally
bright at all viewing angles, the surface is matte, and we have diffuse reflection.
We have mixed reflections if some of the incident light is diffusely reflected and
some undergoes specular reflection. The reflection coefficient of a surface gives the
fraction of the incident light reflected by that surface. Reflection coefficients for
some materials are listed in Table 14-12.
An extended source is a source whose wavelength is in the retinal hazard range,
400–1400 nm, and whose dimensions are large relative to the viewing distance. In
this case, the source can be resolved into a geometrical image on the retina (which
cannot be done with a point source). Typically, direct viewing of laser diodes are
extended sources; and diffuse reflections may be extended sources. For purposes of
laser hazard control, we have an extended source when the source subtends a viewing
angle >1.5 × 10 −3 radians. As long as the eye views a reflection of an extended source,
the intensity of the light energy on the retina is independent of the viewing distance,
since the light intensity variation with distance is exactly compensated by the change
in the subtended viewing angle (Fig. 14-8). The relationship among the size of the
reflection, DL; the viewing distance, r1; and the angle subtended by the eye, α, is
given by
α = DL × cos θV
. (14.16)
r1