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Daylight Tubes Explained

Author: Glenn Harper
Published: July 24, 2013

The simplest definition of a daylight tube is that it’s an artificial light source that mimics daylight in its cool-white output. However, there are different ‘levels’ of daylight tube. Every product mentioned in this article is a daylight tube, but any that look and behave like daylight are set apart and marketed differently, and they’re still rare. The SAD lamp highlighted later is an example.

Before looking at a few daylight tubes and their various advantages, let’s start with a couple of definitions.

What is daylight?

Daylight is the radiation of light at all wavelengths in the electromagnetic spectrum, between and including ultraviolet and infrared. The visible spectrum ranges from about 390 to 780 nm in wavelength, and is bookended by shorter UV and longer IR bands. Ultraviolet and infrared radiation are both invisible to the human eye.

What is full spectrum light?

Full spectrum light is essentially daylight, as defined above, but it’s a term often used erroneously for its easy marketing appeal. Strictly speaking, full spectrum tubes and bulbs should include UV and IR radiation, which is present to a lesser extent in daylight even on an overcast day.

For the sake of simplicity, we can accept the marketing definition of full spectrum light as being any light that radiates at all wavelengths of the visible spectrum - which is the bit in between UV and IR! Incandescent lights fall into this category.

The main benefit of this in artificial lighting is simple: highly reliable colour rendition. With a ‘full spectrum light’, the colour of any given object is represented in the light, and reflects accurately. Street lamps are a good example of the polar opposite; you struggle to discern any colour because their output is near-monochromatic.

An ordinary ‘daylight tube’ is not a full-spectrum tube, because it doesn’t possess these colour qualities - it only emulates daylight in one respect, and that’s in the colour of its cool white light. The rare artificial lights that possess both qualities, i.e. they cover the full visible spectrum and produce a cool white light, have massive appeal for health reasons, or for any colour critical task or display application. Although they’re still essentially daylight lamps or tubes you can be sure they’ll be labelled differently and aimed towards specific markets.

The daylight tube and its advantages

As mentioned, a ‘daylight bulb’ or ‘daylight tube’ is a reference only to the colour temperature of its light, which is generally required to be 5000K or higher in order to match the blue-white light of daylight. No further specification is implied, so it needn’t radiate across the entire visible spectrum, as daylight does.

We can examine the benefits of a basic daylight tube, assuming it to be no more than a cool-coloured lamp, and the benefits of this property alone might pleasantly surprise you.

Daylight tubes and lamps make us feel alert, help us to concentrate, and even when they’re not ‘full spectrum’ and unerringly colour accurate, very often they’ll still display most colours well. They’re a good choice for task lighting. You’re also less likely to suffer from eyestrain and headaches with a daylight lamp, or tube.

Before we look at a variety of daylight tubes, it might be helpful to brush up on colour rendering, and the CRI (Colour Rendering Index):

Colour Rendering Index

The Colour Rendering Index (CRI) is a quantitative measure of a light source’s ability to render colours accurately by comparison to a reference source. The CRI is sometimes referred to as the ‘Ra’, which actually refers to the test method used in establishing the CRI rating, so effectively has the same meaning. Think single-malt whiskey and Glenfiddich.

Often criticised for its leniency, the CRI rating is essentially calculated using eight low-saturation colour samples. The maximum CRI achievable is 100, but because the score is averaged it can easily appear flattering even when certain colours are rendered poorly. It’s worth noting, too, that the CRI does not indicate the apparent colour of the light source, so a score can be high without the light necessarily looking natural.

The lighting industry generally accepts a CRI of 80 to be the threshold for high quality colour rendering, whereas photographers prefer to push this to 90, since it leaves less scope for disaster under the ruthless scrutiny of an imaging sensor. If you’re looking for an extremely natural light source for colour-critical work or health reasons, you basically want the score as near to 100 as possible, which cuts out the averaging illusion - and even then you ideally need to know more (e.g. its colour).

Any light source with a continuous spectrum - one that radiates at every visible wavelength - scores a CRI of 100 by default. That includes daylight, incandescent light, and camera flashlights.

Types of Daylight Tubes

T5 Daylight tubes

The T5 is the most energy-efficient fluorescent tube there is - a tough adversary even for LED. Lyco sell the 35W G.E. T5 High-Output Fluorescent Tube, which boasts a 3090 lumen output of 6500K light and good quality CRI 80 colour rendering. An impressive 30,000 hour lifespan means several years of maintenance-free use from this top-quality lamp.

An alternative to the 35W T5 is the more powerful 49W G.E. T5 High Output Fluorescent Tub, which is available as a 6500K daylight lamp with impressive 4100 lumen output. Again, the spec is rounded off with decent CRI 80 colour rendering and the same mighty 30,000 hour lifespan.

T8 Daylight Tubes

The T8 is second only to the T5 in fluorescent energy-efficiency, though it usually offers the advantage of being cheaper up-front. Lyco sells the 2-foot 18W G.E. T8 Fluorescent Tube in a 6400K daylight version with a 1300 lumen output. This tube is A-Rated for energy, and boasts a long 15,000 hour lifespan. Its CRI 80 score means high quality colour rendering, so it’s ideal for most general-purpose applications and a natural working environment.

The bigger brother of the 18W tube is the 5-foot 58W G.E. T8 Fluorescent Tube, which delivers a mighty 5000 lumens of 6400K light in its daylight version. Both of these T8 tubes use a triphosphor coating for improved light levels, lifespan, and colour rendering.

SAD Lamps

S.A.D. is an acronym for Seasonal Affective Disorder, which is a depressive disorder that occurs most often during autumn and winter months as a result of light deprivation. Symptoms are synonymous with other forms of depression, including lack of motivation, social withdrawal, tiredness, and overwhelming feelings of hopelessness.

One of the treatments for SAD is bright light therapy, and it should be stressed that this generally falls outside the scope of a household lamp. Illumination usually comes from a high-output light box of 10,000 lux illuminance, which is essentially designed to closely replicate daylight in both quality and intensity.

Acting as a preventative measure or supporting role against SAD, Lyco sells the Sylvania T8 SAD Fluorescent Tube, which is part of Sylvania’s Activa range. This light crucially combines a 6500K daylight temperature with an extremely high CRI 96 rating, signifying excellent colour rendering.

Aside from its potential health benefits - for SAD sufferers and non-sufferers alike - this Sylvania tube is a great choice for colour-critical environments such as high-end retail, photo labs, art studios, galleries, and beauty salons. In addition to a wonderfully natural output, you’ll enjoy an average 20,000 hour lifespan from this healthy light.

Daylight choice

Very few lamps exist that can emulate daylight on the important dual axes of cool colour temperature and even distribution across all wavelengths of the visible spectrum. Incandescent technology comes close, but falls short through its relative weakness at shorter wavelengths and strong bias towards red.

A refined halogen lamp has long existed for highly colour-critical applications, but can only be acquired from a specialist source. The spectral distribution of this lamp is modified for better representation at shorter (violet and blue) wavelengths, which brings it closer to the quality of daylight. It is popular amongst museums, galleries, artists, and photographers, and one of its few enemies is the looming shadow of LED research and development!

SAD lamps - which are daylight lamps with significantly improved colour rendering - offer a better, healthier, and more colour-accurate way of working for many people, in addition to potentially alleviating mental health problems.

For night-time outdoor environments, where colour rendering has traditionally been poor, relatively new lamp types such as the ceramic metal halide can provide light that is more akin to daylight. The G.E. 150W Double Ended Ceramic Metal Halide, for instance, offers extremely good CRI 90 colour rendering in its cooler 4200K Natural Deluxe version. Such lamps are useful as a crime-prevention measure - they make detailed description easier - and they’re helpful for identifying and allowing colour-coded safety signs and road markings.

The light at the end…

Hopefully this article will have helped illustrate the various levels of daylight tube and lamp. For a healthier and more comfortable working environment, a basic daylight tube will often be a good investment. When you need a daylight tube to be as close to daylight as possible, either for health or colour-critical reasons, you need to pay close attention to the CRI and preferably a spectral power distribution chart too (Osram-Sylvania include these in their SAD literature). Such a graph provides direct indication of colour rendering and quality, without the spin!

Why not take a look at our current range of daylight tubes.

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