Light transmission or light shading: which number is on your spec sheet?

This is the single most common misreading in backlit textile buying, and it is not the buyer's fault. The two figures come from different test traditions. Transmission is the older optical convention and still appears on woven backlit sheets. Light shading comes out of the Chinese textile standard FZ/T 62025-2015 and appears on most knitted display fabric sheets. Both are legitimate. Neither sheet is obliged to tell you the other number exists.

How do I convert light shading to light transmission?

Subtract it from 100. A fabric at 61 % light shading transmits about 39 %. The conversion is approximate rather than exact — the two tests do not use identical apertures or light sources — but it is close enough to rank materials against each other, which is what the number is for.

Here is Epic's own range normalised to one column, light through, so it can be read top to bottom:

Fabric	As published	Light through
Rio	41 % light shading	59 %
Halo	48 % light transmission	48 %
Lumex	52 % light shading	48 %
Tesa	60 % light shading	40 %
Aureline	61 % light shading	39 %
Eos	100 % light shading	0 %

Read the middle column on its own and Halo looks like it sits between Rio and Lumex on brightness. It does not — it ties with Lumex, and Rio is clearly the brightest of the six. Read the published figures as a single list and you would also conclude Eos is the best backlit fabric in the range, when Eos is a blockout: it is designed to pass nothing at all.

Is a higher light transmission always better for a light box?

No, and this is where the number stops being a ranking and starts being a specification.

Light passing through the fabric is light that was not scattered by it. Scattering is what hides the LED array. A fabric that transmits a great deal of light efficiently can also telegraph the position of every diode behind it, which shows up as a regular grid of bright patches across a dark image area. The industry calls them hot spots, and they are a function of three things together: how much the fabric diffuses, how deep the box is, and how closely the LEDs are spaced.

The practical consequence is that transmission has to be matched to fixture depth. A shallow frame — 50 mm or less, which is most retail light boxes now — has very little distance in which light can even out before it reaches the face. It needs a fabric that scatters more, which usually means transmitting less. A deep box, or an edge-lit frame where light enters from the sides and bounces, can take a brighter, more open fabric and benefit from it.

So a fabric at 39 % light through is not a worse fabric than one at 59 %. It is a fabric for a different frame.

Why do two fabrics with the same transmission look different?

Because transmission is a single scalar and the eye is reading several variables at once.

Halo and Lumex both land at 48 % light through, and they do not look the same lit. Halo is a 180 gsm woven construction; Lumex is a heavier knit. The weave geometry changes how directional the transmitted light is — a woven structure with a regular pitch can produce a faint moiré against a regular LED array that a knit will not. Lumex also carries a measured yellowing figure of ΔE 1.84, against Tesa's ΔE 0.50, and yellowing is far more visible backlit than it is in reflected light, because every bit of the illumination is passing through the discoloured layer rather than bouncing off the top of it.

Ink load matters as much as either. A dense black in the artwork can cut local transmission by most of the fabric's rated figure, which is why a graphic with large solid dark areas and small bright ones is the hardest thing to light evenly. Two fabrics can measure identically on a blank swatch and separate completely once there is artwork on them.

What should I ask a supplier if the spec sheet only gives one figure?

Four questions, in this order:

1. Which figure is it — transmission or shading? If the sheet does not say, the test method usually gives it away. FZ/T 62025-2015 is a shading test.
2. What tolerance band? A figure quoted as "62 % (±5 %)" means anything from 57 to 67 % ships as on-spec. Across two production lots that is a visible difference in a matched pair of panels.
3. Was it measured on white, or on printed material? It will have been measured on white. Plan for the printed figure to be lower.
4. Can I have a sample to test in my own frame? This is the only question that actually settles it. Hot-spotting is a property of the fabric, the depth and the LED pitch together, and no number on a sheet can predict the combination.

The short version

Convert everything to one column before you compare anything. Then treat the result as a specification to match to your fixture depth, not a score to maximise. Then get a sample and put a light behind it, because the number tells you which fabrics are worth sampling and nothing more than that.

Every figure in this article is from the measured test data on our own spec sheets, with the test method named. If you want the sheets, they are on each product page — and if you want yardage to put in front of your own LEDs, ask us for a sample.