Do halogen cover lenses need to be a specific type of glass? Here is the fixture.

enter image description here

I won’t be using more than a 20 watt bulb.

Motivation: I found an old lamp that takes JC type bi-pin bulbs. I can’t find a 90+ CRI LED that fits in the fixture, so I want to use halogen. Problem is, the protective lens is missing, and it’s a nonstandard size I haven’t been able to find a proper replacement for, so I’m looking to have something made custom.

  • 2
    Check a chemistry supplier for a "watch glass" before you assume it's a custom size. en.wikipedia.org/wiki/Watch_glass Might want to use the mm side of your ruler.
    – Ecnerwal
    Jan 14, 2023 at 17:26
  • @AloysiusDefenestrate, if you know where I can find a 90+ CRI 6.35 mm bi-pin LED that has the same shape base as a standard JC halogen, that would be great! So far, the only 90+ CRI 6.35 mm bi-pin LED I found had a large round base that didn’t fit into the fixture: EmeryAllen EA-GY6.35-4.0W-001-279... amazon.com/dp/B07N2YV6HW
    – Evan
    Jan 14, 2023 at 19:01
  • @Ecnerwal that’s the wrong pin type (G9) unfortunately.
    – Evan
    Jan 14, 2023 at 19:06
  • That fixture may be from the before-glass-shield era, so there may not have been a protective lens. I don't see any offsets to hold such a lens above the flat metal to allow for cooling airflow.
    – Armand
    Jan 14, 2023 at 19:08

2 Answers 2


Borosilicate (generic of "Pyrex® brand) would be the best bet.

That's used for laboratory glassware that takes direct flame heating - it has low thermal expansion.

The lamps of this sort I've seen and owned appeared to use off-the-shelf watch glasses (as suggested in comment) - it would appear that a 50 or 55mm would be the correct size, but that's eyeballing the wrong side of the ruler.

"Flint glass" (the other usual option) would be less suitable.

The fact that you had part of a lens in there indicates that it was not tempered, as a tempered cover glass would have broken to bits, rather than leaving appreciable parts intact.


  • Do you know whether standard halogen lenses are made of borosilicate? All the lenses I see for MR16, for example, are described only as ‘glass’.
    – Evan
    Jan 14, 2023 at 21:48
  • I found at least one supplier who did admit to borosilicate. Many don't say. Borosilicate is a type of glass, so those less detailed could be using it, or using something else. Since it is highly resistant to thermal shock, it would be a most suitable choice.
    – Ecnerwal
    Jan 14, 2023 at 22:08
  • Halogen bulbs are usually synthetic quartz (AKA fused silica, quartz glass, etc.). The problem with that is that it transmits much more UV than normal glass or borosilicate. The glass window (or sometimes lens) on the front blocks the UV
    – Chris H
    Nov 18, 2023 at 18:27

If that fixture was made for LED bulbs you may not be able to use halogen. The voltage to the bulb may be different than each bulb needs, also the fixture may not be able to handle the extra heat that a halogen bulb creates. Check the comments and see what Aloysius Defenestrate has posted for LED bulbs.

  • 1
    Zoom in and read the text - it's clearly a halogen fixture. 12V, 30W Fire and Burn hazards. The linked list of bulbs does not seem to be limited to 90+ CRI, CRI was either not specified or at most >85 on any I bothered to check - though of course shopping for 90+CRI LEDs would be a shopping question.
    – Ecnerwal
    Jan 14, 2023 at 18:42
  • as I zoom in , you may want to notice the first word in my answer: "IF"... The bulb in the pic is clearly an LED, and the poster was initially looking for LEDs
    – RMDman
    Jan 14, 2023 at 19:03
  • @RMDman this fixture was made for halogen. I believe it was manufactured before 1980. I have an LED in there now (replacing a burned out halogen) until I can find a replacement lens.
    – Evan
    Jan 14, 2023 at 19:12

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