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What are dichroic lamps?

Author: Glenn Harper
Published: August 1, 2013

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The term ‘dichroic lamp’ almost always refers to low-voltage MR11 or MR16 halogen spotlights. The interior surface of these lights is designed as a multifaceted reflector (MR), with the purpose of gathering up the widespread light of the burning tungsten filament and projecting it forward through the front of the lamp.

Invariably the surface of this reflector is manufactured in one of two ways; it’ll either have an opaque aluminium coating or a dichroic coating. The purpose of an aluminium coating is uncomplicated: it projects as much light as possible forward without discriminating between visible light and invisible UV or IR radiation, either of which can potentially be harmful.

A dichroic coating is essentially a thin layer of non-metallic film, sometimes referred to as interference film, which reflects visible light from the filament forward whilst filtering infrared radiation and allowing it to pass through the back of the lamp. Since IR radiation is a significant source of heat, the net effect of this is to make the beam much cooler.

Dichroic compatibility

When buying or installing a dichroic lamp you first have to ensure that the light fitting or lamp-holder can dissipate the back-firing heat. Any recessed or enclosed luminaires that cannot accommodate such a lamp should be marked with the IEC 605598 ‘No Cool Beam’ symbol. If your light fitting is labelled in this way you’ll need an aluminium reflector lamp.

Benefits

The beam of a dichroic lamp is significantly cooler than other halogen spotlights, which extends its usefulness drastically for displaying heat-sensitive objects such as paintings, photos, leather goods, food, and wine. One such lamp is the superb Osram Decostar Energy Saving Halogen MR16 51S, which uses a dichroic coating to reduce heat in the beam by a remarkable 66%. This lamp further manipulates IR radiation with IRC technology, which reflects infrared heat back onto the filament to increase energy efficiency.

A second potential benefit of a dichroic coating is that it can be used to remove longer [redder] wavelengths of the visible spectrum to create a halogen spotlight with an unusually cool temperature. This has some appeal, because it’s ordinarily uncommon for a filament lamp to output anything other than a warm light, but there is a trade-off in colour accuracy and a lower CRI score. Most dichroic lamps, like the great-quality Philips Accentline Halogen GU5.3 Spotlight, remove only IR radiation and maintain their maximum CRI 100 rating for colour rendering.

Low-voltage MR11 or MR16 halogen dichroic lamps have other inherent advantages that complement their IR filtering properties admirably. With a relatively compact filament they are optically very controllable with minimal spill light, and deliver a focused, crisp beam. What’s more, they’re very affordable, and rarely more so than with the bargain-priced Lyco Halogen MR16, which is dimmable, highly colour-accurate, and might last you two or three years – all for mere pence! For mood-enhancing effect you might try some multi-coloured accent lighting with Lyco’s glass-fronted Colour Halogen MR16s.

Other forms of dichroic lamp

Though dichroic lamps are mostly low-voltage, they can occasionally be found as mains spotlights. The common GU10 reflector is really an MR16 lamp with a ‘twist and lock’ 240V mains-connectable base. Cool-beam versions of the GU10 can be found with a dichroic coating, although they are known as ‘GZ10’ lamps. Lyco sells the dimmable Sylvania Hi-Spot ES50 Halogen GZ10, for instance, which is a great choice of mains-powered spotlighting around heat-sensitive objects. Again, great care needs to be taken in ensuring this lamp is installed into a fitting with sufficient heat dissipation.

The future of cool-beam halogen

The very need to handle unwanted heat in a lamp is, of course, indicative of poor energy-efficiency, and that places dichroic lamps in something of a precarious position. In Australia the common low-voltage 50W MR16 has already been subject to a phase-out, and there have been one or two reports in the UK of its imminent European demise. When this does come to fruition – and it seems only a matter of time – it is believed the more energy efficient IR-reflecting and Xenon-filled 12V halogen spotlights will remain available (the Osram Decostar previously highlighted being an example).

MR16 lamps have existed for over 30 years, and the threat of their extinction has caused some disquiet among lighting designers worldwide. With the more energy-efficient models seemingly safe for at least another three years or more, we might in that time expect LED alternatives to have been further improved and for their prices to have finally toppled. In the meantime, you can still reap the benefits of a crisp, bright, vivid, colourful, pretty cool halogen source of light!

For more advice, inspiration and news take a look at our Lighting Advice section.