How to lume: The Bell & Ross BR-X5 LUM lume story

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We reviewed the Bell & Ross BR-X5 Green Lum last year, and were impressed on how BR achieved this lume on the watch. We investigated and here are more details.

How to lume: The Bell & Ross BR-X5 LUM lume story

The Bell & Ross BR-X5 Green Lum is an interesting watch. The watch case is fully luminous, and truly glows in the dark. Read our review for the low down on the watch, but here is a quick summary:

  • Limited Edition of 500 pieces
  • 41mm nominal diameter
  • Caliber BR-CAL.323 Automatic mechanical movement sourced from Kinessi
  • Case is made of LM3D material – a resin, quartz fibre, luminescent material and dye
  • The LM3D is as scratch resistant as steel, but is 84% lighter for the same volume
  • Luminiscence can last up to 10 hours when the LM3D is fully charged
  • Takes about 20 minutes of light (either in the sun or indoor lighting) to fully charge the LM3D

The technical challenge

We had an interesting discussion with the technical folks at Bell & Ross, and got an appreciation on the level of difficulty in creating this watch. The initial approach was to find a material as luminous as SuperLumiNova but encased in ceramic.

This presented difficulties, as luminescent products do not withstand the temperatures required to agglomerate the ceramic. And the research team directed their focus on a kind of luminous carbon fibre. They wanted a homogeneous material with no visible pattern or crystalline structure.

The base material selected is a quartz fibre. This is a 99% silica fibre which exhibits almost perfect transparency, much better than fibreglass. To bind the fibres together, a carbon expoxy which is also transparent is chosen. The epoxy has good adhesion to the fibre. This has been field tested in other applications including in the field of aeronautics.

Formed into a block of material with the quartz fibres, the block is homogenous and resistant to shocks. It is then machined into the required shapes. It was mechanically similar to carbon fibre and did not alter its appearance when exposed to UV light. To this solid block, luminescent material is added to make it glow in the dark. The material, called LM3D is exclusive to Bell & Ross.

The next task to tackle is to introduce colour into the material. Colour pigments absorb the lume and takes away from brightness in the dark. The right balance between the pigments for colour and luminescence is needed. And a special process is needed to obtain a homogenous material after polymerisation. This is to ensure that the colour and luminescent material is evenly distributed within the structure so that the resultant lume does not have bright or dark spots. And the final lume and colour had to perfectly align with the iconic C5 SuperLumiNova colour that BR uses for the indices and hands.


The result is for us to see. The material is homogenous in colour and is completely luminous. In our lume photograph below was taken with about 10s under a UV torchlight. Note that the SuperLumiNova indices and hands are fully charged, but it takes a bit more time to fully charge the case.

But when fully charged, the lume is very even.

This photograph was taken with an iphone, and due to the ambient lighting, and iOS’ white balance, the colour is not accurate. But the lume on the case is completely even.

The material is as light as carbon fibre, and is stable over time. The material can also be polished to remove scratches.

Interesting result from a technically interesting diversion from traditional watchmaking.


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