What is prism cleaning?
Removing rubber, dirt and film from the prism of an airfield light so the beam leaves at full strength and in the right direction. In an inset light the prism is the window to the pilot, and it sits right in the traffic.
The prism bends the light into a shaped beam and, in an inset light, also forms the window at the top of the fixture, flush or nearly flush with the pavement. Landing tyres lay down rubber, and dust, oil and film add to it. Every layer blocks and scatters light, so the intensity falls and the beam can lose its shape, with nothing wrong in the lamp or LEDs.
1Make it safe first
Isolate and lock out the circuit before touching the fixture, as your site procedure requires. A lamp-based fixture can be hot, so let it cool. A prism sits in a runway or taxiway, so work only with the right permission and under the site’s safety arrangements for the movement area.
2Clean the prism the right way
Brush off loose grit first so it does not scratch the glass. Then use the cleaner and cloth the manufacturer approves, usually a soft, lint-free cloth with water or a mild cleaner, and the approved method for rubber. Clean any channel or opening in front of the prism as well, because a blocked channel cuts the beam as surely as a dirty prism. Do not scrape, and do not use abrasive pads or strong solvents: a scratched or crazed prism scatters light for good.
- Brush off loose grit before wiping
- Clean the channel in front of the prism
- Use the maker-approved rubber-removal method
- Scrape or use abrasive pads
- Use strong solvents on the prism
- Let grit or water fall past the seal
3Inspect and record
With the prism clean, look for cracks, chips, scratches, a damaged gasket or O-ring, loose retaining bolts, water inside and a shifted alignment. The prism is held by a retaining ring and bolts, so refit anything you removed and check it as the maker instructs. Optical parts are normally keyed, so replace a damaged prism only with the correct part. Then confirm the light works and record the condition and the work done.
| Item | What to do |
|---|---|
| Prism surface | The window the beam leaves through; keep clean and unscratched |
| Loose grit | Brush off first so it does not scratch the glass |
| Rubber deposits | Build up in touchdown areas; approved method only |
| Channel in front | Clear it so the beam is not blocked |
| Gasket or O-ring | Check while cleaning; keep water out |
| Retaining ring and bolts | Refit and check as the maker instructs |
| Prism condition | Cracks, chips or crazing mean report it and replace with the correct part |
| Circuit and area | Isolated, with permission and safety arrangements in place |
| Record | Note condition and what was done |
Frequently Asked Questions
Q1Why does the prism of an inset light need cleaning?
The prism is the window the beam leaves through, and it sits at pavement level. Rubber from landing tyres, dust, oil and film settle on it and block and scatter the light, so the fixture looks dim even when the lamp or LEDs are fine.
Q2Is cleaning a prism the same as cleaning a lens?
The aim is the same, a clear optical surface, but a prism sits flush in the pavement and collects rubber and traffic residue. You also clean any channel in front of it and must not let dirt or water get past the seal.
Q3Should the circuit be isolated before cleaning a prism?
Yes. Isolate and lock out the circuit as the maintenance plan and site rules require, and work on the movement area only with the right permission and safety arrangements.
Q4What should not be used to clean a prism?
Do not scrape it or use abrasive pads or strong solvents. Use the cleaner, cloth and rubber-removal method the manufacturer approves, because a scratched or crazed prism scatters light for good.
Q5What should be checked after the prism is cleaned?
Look for cracks, chips, scratches, a damaged gasket or O-ring, loose retaining bolts, water inside and a shifted alignment. Replace a damaged prism only with the correct part, then record the condition and the work done.
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