Prismatic lensesincorporate a pattern of
small prisms or other refractive elements to
reduce the brightness of the luminaire and
inhibit direct glare. But almost all fluorescent
luminaire lenses fail to reduce their lumi-
nance sufficiently to provide visual comfort
and prevent bright images in VDT screens.
The excessive contrast between the lens and
the ceiling plane also creates distracting
reflections.
Egg-crate louversare made of intersect-
ing straight-sided blades that reduce lumi-
nance by blocking light rays that otherwise
would emerge at glare angles. They are
made of translucent or opaque plastic or
painted metal. Egg-crate louvers are ineffi-
cient in transmitting light, controlling glare,
and preventing VDT screen reflections.
Parabolic louverscontrol luminance pre-
cisely; they consist of multiple cells with par-
abolic reflectors and a specular or semi-
specular finish. The cells range in size from
½ in × ½ in to 1 ft × 1 ft.
Small-cell parabolic louvers reduce
luminance, but are inefficient in light output.
To maximize efficiency, they often have a
highly specular finish, which may cause such
a low luminance at the ceiling plane that the
room seems dim and depressing.
Deep-cellopen parabolic louvers offer
the best combination of shielding and effi-
ciency.
To avoid reflected glare in VDT screens,
IESNA recommends that average luminaire
luminance be less than
850 cd/m^2 at 55° from nadir
350 cd/m^2 at 65° from nadir
175 cd/m^2 at 75° from nadir
A manufacturer’s luminaire photometric
report should include a luminance summary
that tabulates brightness values at angles
above 45° from nadir. This summary may be
used to evaluate the suitability of direct
luminaires in offices with VDTs.
Although use of the footlambert (fL) is
discouraged, some manufacturers still pro-
vide average luminance data in fL instead of
cd/m^2. To check compliance with these limits,
multiply the fL values by 3.42 to determine
cd/m^2.Spacing criterion
Manufacturers will sometimes publish the
luminaire spacing criterion (SC) for their
downlight equipment. This is an estimated
maximum ratio of spacing to mounting-
height above the workplane in order to pro-
duce uniform, horizontal illuminance. SC is a
low-precision indicator; its purpose is to aid
the designer in quickly assessing the poten-
tial of a downlight luminaire to provide uni-
form illumination of the horizontal plane.
SC values are sometimes assigned for
uplights, but they are rarely assigned to wall-
washers, object lights, task lights, or multidi-
rectional lights because these luminaires are
not intended to provide uniform, horizontal
illuminance.Luminous ceilings
A luminous ceiling also provides direct,
downward distribution. It consists of a plane
of translucent glass or plastic—often the
size of the entire room—suspended below a
regular grid of fluorescent lamps. The sus-
pended element becomes the finished ceil-
ing. This technique, popular in the 1950s
and 1960s, provides uniform, diffuse, ambi-
ent light (figure 12.18).
The cavity above the luminous plane
must be free of obstructions and all sur-
faces are to be finished with a high-
reflectance (80 to 90 percent), matte-white
paint. Luminous ceilings share the same
drawback as indirect lighting—they light
everything from all directions, with no shad-
ows or modeling, giving the gloomy effect of
an overcast sky.INTERIOR LIGHTING FOR DESIGNERS