# Visible Light Transmission: The Glazing Number People Forget

> Visible light transmission is the fraction of visible daylight that passes through a glazing unit, expressed as a percentage or decimal. It governs how bright the interior is and how much daylight reaches into the floor plate, and it usually falls as solar control improves — which is the trade-off at the centre of glass selection.

## Why it matters on a project

Glass selection is usually argued on solar control, because that is the number
tied to air conditioning load. Visible light transmission is the counterweight,
and leaving it out produces buildings that are thermally efficient and
unpleasant to occupy.

The cost of getting it wrong is not just comfort. Lights running all day are an
energy cost, a maintenance cost and an additional heat load, which partly
cancels the saving that the dark glass was chosen to deliver.

## The three numbers that describe a glazing unit

| Number | What it describes | Direction |
| --- | --- | --- |
| U-value | Conducted heat flow | Lower is better insulated |
| SHGC | Fraction of solar energy transmitted | Lower reduces cooling load |
| VLT | Fraction of visible daylight transmitted | Higher is brighter inside |

The relationship that matters commercially is **VLT divided by SHGC** — the
light-to-solar-gain ratio. It expresses how much daylight the glass delivers per
unit of solar heat admitted, and it is the single most useful figure for
comparing high-performance coatings against each other.

Luminous and solar characteristics are determined under **EN 410**.

## Reading it in context

A glazing choice only makes sense against four other things:

- **Orientation.** The sun a facade receives varies enormously by direction.
- **Shading.** External shading, fins and overhangs change the problem before the
  glass does.
- **Floor plate depth.** Daylight reaches only so far in; beyond that, VLT is
  irrelevant to that part of the floor.
- **Interior layout.** Enclosed rooms along the facade block daylight from
  everyone behind them, whatever the glass does.

Interior illumination practice is set out in **IS 3646**, and daylighting and
envelope provisions appear in **ECBC 2017**.

## Common mistakes

- **Optimising SHGC alone.** Produces a dark building and shifts energy to
  lighting.
- **Maximising VLT on a sunlit elevation.** Glare closes the blinds permanently.
- **One glass specification for all four orientations.** Convenient, and a
  compromise on every side.
- **Ignoring the light-to-solar-gain ratio.** It is the number that actually
  distinguishes a good selective coating from an ordinary tint.
- **Forgetting reflectance outward.** Strongly reflective glass can be a
  nuisance to neighbours and to traffic.

## Standards referenced

Luminous and solar characteristics of glass to **EN 410**; interior illumination
practice in **IS 3646**; envelope and daylighting provisions in **ECBC 2017**.
Glass selection and compliance for a specific building must be established by
the project's facade and energy consultants.

## Frequently asked questions

### How does VLT relate to SHGC?

They usually move together — coatings that block solar energy tend to block visible light too. The useful measure of how well a glass separates them is the light-to-solar-gain ratio, VLT divided by SHGC. A higher ratio means the glass is letting daylight in while keeping heat out, which is exactly what a selective coating is designed to do. Comparing that ratio is more informative than comparing either number alone.

### What happens if VLT is too low?

The interior goes dark and artificial lighting runs all day, which moves energy consumption from cooling to lighting rather than reducing it, and adds the heat those lights produce back into the space. It also affects how occupants experience the building — a heavily tinted facade can make an office feel gloomy even when illuminance readings are adequate.

### Is a high VLT always better?

No. High transmission on a strongly sunlit elevation produces glare at desks and heat at the perimeter, which is why blinds get closed permanently — at which point the daylight is lost anyway and the glass specification has achieved nothing. VLT should be set alongside orientation, shading and the internal layout, not maximised in isolation.

### Does VLT differ across a building?

It can, and often should. Elevations facing different orientations receive very different sun, so a single glass specification applied to all four sides is a compromise everywhere. Varying the specification by orientation is more work at design stage and generally produces a better building, though it does need careful management to avoid the wrong glass reaching the wrong elevation on site.

### How does it affect exterior appearance?

Low VLT glass usually reads as dark or strongly reflective from outside, which is an architectural decision as much as a technical one. Highly reflective facades can also cause glare for neighbouring buildings and roads, which is worth considering at design stage rather than after complaints.

## Sources

- [ECBC 2017](https://beeindia.gov.in/) — Building envelope and daylighting
- [IS 3646](https://standards.bis.gov.in/website/published-standards/department-wise) — Code of practice for interior illumination
- [EN 410](https://standards.cencenelec.eu/ords/f?p=CEN:105) — Glass in building — luminous and solar characteristics

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Source: https://hagerstone.com/glossary/visible-light-transmission