touchdown zone lights, the light beam is offset 4 degrees toward the runway centerline. The
aiming of touchdown zone lights may be judged by viewing the barrettes on either side of the
runway while standing on the centerline. When viewing the barrettes on either side of the
runway from some distance, any light appearing dimmer or brighter than the lights next to it may
be improperly aimed and should be checked.
d) Light Cleaning and Sealing. In pavement lights gradually get dirty internally, and
the internal optical surfaces should be cleaned when the light is disassembled for relamping or
maintenance. Sandblasting may be used to clean rubber deposits off the casting after all
removable parts have been taken off. Use a cleaning solution that does not leave a residue after
drying. When relamping a light, be careful to handle the lamp by the leads only; fingerprints on
the glass assembly will shorten lamp life. Lamps in brackets should be mounted according to
manufacturer's recommendations; using the wrong lamp or mounting it improperly can
drastically reduce the light output of the fixture. When reassembling the light, replace all gaskets
and O-rings exposed during the relamping process. Examine the optical prism to make sure that
the sealer around the edges is in good shape. If the optical prism is cracked or badly pitted, it
should be replaced.
e) Reinstallation. When mounting an in pavement unit on its base, care must be used
to be sure that a watertight seal is obtained. The fixture connections to the series circuit should
be sealed with two layers of plastic tape or with heat-shrinkable sleeving. Heat-resistant varnish
may also be used to improve the sealing of the connection and protect the tape from heat. Be
sure the gasket and its mating surface are free of sand or grit; this is a common fault in servicing
that allows moisture to enter. Graphite compound or gasket cement may be used on the gasket
surfaces to ensure a watertight seal. Securely tighten all fixtures to the manufacturer's specified
torque. The bolts and threads should be cleaned, and the threads may be coated with a securing
compound such as Locktite 242, or equivalent.
f) Water Removal. The procedure for removing water from the base of in pavement
lights and preventing reentry is similar to that described in par. 5.5.2.h(2) for runway edge lights.
If the fixture itself leaks, renew all gaskets and sealants.
g) Photometric Measurements. Photometric measurement of in pavement lights is the
most direct way of determining if they are emitting the specified amount of light. The procedure
presented below, using a photographic 1 degree spotmeter, may be used when the light is
installed or in the shop. Photometric measurements of installed in pavement lights help
determine if the window area should be cleaned, or if the light is in need of maintenance. The
photometric procedure presented below relies on the comparison of relative values of light.
While it is useful for determining the relative performance of the light, care should be used when
comparing dissimilar lights, as the standard values established may not apply.
(1) Energize a new light at maximum intensity (6.6 amperes). Take a reading with
the spotmeter at a distance where the window area of the light unit just fills the 1 measuring
spot of the meter. The observer should move the meter vertically and horizontally far enough to
ensure that the maximum reading is observed (center of the light beam). This maximum value,
usually in exposure value (EV) units, should be recorded and used as the standard value to which
field measurements are compared.
(2) Next take photometric readings of the lights to be sampled, using the technique
described in subpar. (1). The lights should be set on the maximum intensity step. If the EV
reading of the spotmeter is more than 2/3 EV lower than the reference value determined in
subpar. (1), the light is in need of servicing. If a scale other than EV is used, the light should be
serviced when it emits less than 70 percent of the light it emitted when new.