Visible light transmission (VLT) is the percentage of visible daylight that passes directly through a glass or glazing unit, measured on a scale from 0% (opaque) to 100% (perfectly clear), covering the visible spectrum of roughly 380 to 780 nanometres. A glass rated VLT 60% lets 60% of daylight through and blocks or reflects the other 40%. It is one of the three core optical-thermal properties used to specify architectural glass, alongside Solar Heat Gain Coefficient (SHGC) and U-value, and it directly determines how bright, glary or dim an interior feels.
VLT is a property of the entire glazing assembly, not just a single pane, so an insulated glass unit (IGU) with two 6 mm panes, a coating and a spacer will report a lower VLT than a single sheet of the same glass. Because it governs daylight, glare and the need for artificial lighting, VLT is a central metric in green-building and energy codes. In a hot, high-sunlight city like Hyderabad it must be balanced against heat gain rather than maximised on its own, which is why every serious glass facade project starts with a VLT and SHGC target before a single panel is ordered.
This guide explains how VLT is measured, the typical values for each glass type sold in Telangana and Andhra Pradesh, how VLT interacts with SHGC and U-value, and how to pick the right number for Gachibowli, Kokapet, Madhapur and other Hyderabad micro-markets. If you would rather have a specialist do the calculation for you, our team can help you get a free quote with performance figures included.
How Is VLT Measured and Reported?
VLT is measured as the ratio of visible light passing through the glass to the visible light striking it, weighted to the sensitivity of the human eye, and reported as a percentage or decimal (e.g. 0.62 = 62%).
The figure is derived from spectrophotometer readings across the visible band and calculated per international methods; manufacturers such as Saint-Gobain, AIS, Guardian and Sisecam publish it on product datasheets alongside SHGC and U-value. When you commission a structural glazing facade, the fabricator should hand you these datasheets so the specified VLT is traceable to a tested product, not a guess.
Key points to understand when reading a VLT specification:
- Higher VLT (70-90%) means a brighter, more transparent facade with more daylight and often more glare.
- Lower VLT (8-30%) means a darker, more private facade that cuts glare and, with coatings, solar heat.
- VLT falls as you add panes, coatings, tints and interlayers, so an IGU always transmits less than a single lite of the same base glass.
- Two glasses can share the same VLT but have very different SHGC, so VLT alone never describes thermal performance.
- Exterior VLT and interior visibility are not the same thing; a reflective coating can give a mirror-like outside appearance while still passing usable daylight inward.
Typical VLT Values by Glass Type
VLT varies widely by glass type, from about 88-90% for clear float glass down to under 10% for heavily tinted or reflective glass. The numbers below are for common 6 mm products and shift with thickness and coating.
- Clear float / annealed glass (6 mm): approximately 88-90% VLT.
- Clear toughened (tempered) glass to IS 2553: roughly 88-89% VLT, toughening barely changes light transmission.
- Tinted / body-tinted glass (grey, bronze, green, blue): about 30-70% VLT depending on colour and thickness.
- Reflective (metallic-coated) glass: approximately 8-40% VLT, prized for glare and privacy control on west-facing reflective glass facades.
- Low-emissivity (low-e) coated glass: commonly 40-70% VLT while sharply cutting heat and U-value.
- Double-glazed DGU facade with low-e: often 50-65% VLT combined with much lower SHGC and U-value than single glass.
- Laminated glass with a clear PVB interlayer: typically 85-88% VLT, as the interlayer absorbs little visible light.
- Spandrel or back-painted glass used to conceal floor slabs: effectively 0% VLT by design, opaque on purpose.
Because the same brand name can cover several coatings, always confirm the exact product code on the datasheet rather than assuming a category-level number.
What Is the Difference Between VLT, SHGC and U-Value?
VLT, SHGC and U-value are three separate glass properties that must be read together, because a single number cannot describe how glass performs.
- VLT (Visible Light Transmission): fraction of visible daylight transmitted, 0-100%; governs brightness and glare.
- SHGC (Solar Heat Gain Coefficient): fraction of total solar heat (visible plus infrared) that enters, 0-1; governs cooling load.
- U-value: rate of non-solar heat conduction through the glass in W/m2K; lower means better insulation, which matters most for air-conditioned interiors.
The relationship most useful for hot climates is the Light-to-Solar-Gain ratio, LSG = VLT / SHGC. An LSG above 1.25 marks a spectrally selective glass that lets in daylight while rejecting heat; premium low-e products reach LSG values of 1.7 or higher.
Example: a glass with VLT 60% and SHGC 0.27 has LSG = 2.22, delivering good daylight with low heat gain, which is close to ideal for a Hyderabad office facade. By contrast, an old bronze tinted single glass might show VLT 50% and SHGC 0.62, an LSG of just 0.81 that loses daylight AND admits heat, the worst of both worlds.
When you specify curtain wall glazing, insist on all three numbers plus LSG on the tender document so bidders cannot quietly substitute a cheaper, hotter glass with the same VLT.
How Do You Choose VLT for Hyderabad's Climate?
For most Indian commercial facades a VLT of 40-60% offers a practical balance of daylight, glare control and solar heat rejection, especially in Hyderabad where clear-sky solar radiation is high for eight to nine months of the year and pre-monsoon April-May temperatures routinely cross 40 degrees C.
Hyderabad's climate adds three local pressures beyond raw heat: long dry spells that coat glass in dust and lower effective VLT, a sharp monsoon glare when overcast skies scatter bright light, and intense low-angle sun on east and west elevations. All three argue for pairing a moderate VLT with a strong low-e coating rather than chasing maximum daylight.
Practical guidance for specifying VLT by orientation:
- East and west facades in Gachibowli, Kokapet and the Financial District face intense low-angle sun; favour lower-VLT, low-SHGC glass with external shading or aluminium louvers.
- North-facing glazing receives diffuse light; higher VLT (60%+) can maximise daylight with limited heat penalty.
- South facades in Hitec City and Madhapur benefit from horizontal shading plus mid-range VLT around 45-55%.
- Retail shopfronts and lobbies in Kondapur often want VLT above 60% for visual openness; offices target 40-55% to limit screen glare.
- Always pair the VLT choice with SHGC and U-value targets rather than selecting on daylight alone.
Dust matters more than most designers expect: a facade cleaned quarterly instead of monthly can lose 10-15% of its rated daylight, so accessible, cleanable front elevation glazing protects your VLT investment over time.
VLT and Indian Building Codes (ECBC, NBC, GRIHA, IGBC)
The Energy Conservation Building Code (ECBC) and the National Building Code of India (NBC) 2016 both reference VLT and SHGC when regulating window-to-wall ratio, daylighting and facade energy performance.
ECBC sets maximum SHGC limits that tighten as the glazed area of a wall increases, and it rewards effective daylighting that can dim electric lights near the perimeter. VLT is the property that makes that daylight dimming possible, so a very low-VLT glass can technically pass the heat rule while failing the daylight intent.
Green-rating systems reinforce this balance:
- GRIHA awards points for daylight factor and view access, both driven by adequate VLT.
- IGBC (LEED India lineage) credits daylighting and glare control, rewarding a smart VLT-to-SHGC pairing rather than either extreme.
- Both schemes expect documentation, so keep the manufacturer datasheets that state your VLT and SHGC.
For a code-compliant commercial building in Telangana, the safe approach is a spectrally selective low-e IGU with VLT around 50-60% and SHGC around 0.25-0.30, which satisfies ECBC heat limits while still delivering the daylight that green codes and occupants want. A facade consultancy review early in the design stage is the cheapest way to avoid a costly glass re-order after a code check.
How Does VLT Affect Cost, Comfort and Energy Bills?
Selecting the right VLT lowers running costs by reducing both artificial lighting demand and air-conditioning load, which together dominate energy bills in Indian offices.
- Adequate daylight from a suitable VLT can cut perimeter lighting energy use and is credited under ECBC and green-building schemes.
- Over-high VLT without heat control raises cooling load and generates glare complaints from staff at screens.
- Over-low VLT forces lights on during the day, wasting the whole point of a glass facade.
As a rough Hyderabad guide, plain clear toughened glass runs about INR 130-200 per sq ft supplied, a single reflective or tinted glass around INR 180-320 per sq ft, and a performance low-e double-glazed IGU roughly INR 550-950 per sq ft depending on coating, spacer and thickness. Installed facade structural glazing systems add framing, brackets and labour on top of the glass rate.
High-performance low-e IGUs cost more per square metre than plain glass but typically pay back through lower HVAC and lighting bills over the facade's service life of 25 years or more. Coatings and interlayers that tune VLT also add safety, acoustic and UV-blocking benefits within the same unit, so the premium buys more than daylight control alone.
Common VLT Mistakes to Avoid
Most VLT problems on Hyderabad projects come from a handful of avoidable errors made at the specification stage.
- Specifying VLT alone and letting the supplier pick SHGC; this is how a facade ends up bright but scorching.
- Copying a single-glass VLT number onto an IGU drawing, then being surprised the built facade looks darker.
- Choosing a very low VLT for privacy, then running lights all day and losing the energy case entirely.
- Ignoring orientation and using one glass for all four elevations, so the west facade overheats while the north feels gloomy.
- Forgetting internal blinds and partitions; a glass partition or solid wall behind the facade changes how much daylight actually reaches the room.
- Skipping mock-ups; a small on-site sample under Hyderabad sun reveals colour, reflection and glare that a datasheet cannot.
You can see how we balance daylight, heat and appearance across real elevations in our completed projects, which show how the same VLT logic plays out on offices, showrooms and residences.
How Hakimi Aluminium and Glass Specifies VLT
Hakimi Aluminium and Glass supplies and installs performance glazing with specified VLT, SHGC and U-value for facades and structural glazing across Hyderabad, Secunderabad and the wider Telangana and Andhra Pradesh region.
Our process starts with orientation and use: we map each elevation, set a target VLT and LSG band, then shortlist coated glasses that meet ECBC while delivering the look the architect wants. From clear aluminium doors and windows to high-performance DGU facades, every quotation carries the actual optical and thermal numbers, not just a glass name.
- We share manufacturer datasheets so your VLT and SHGC are traceable and code-defensible.
- We recommend orientation-specific glass rather than one number for the whole building.
- We combine glass selection with shading, louvers and framing for a complete thermal solution.
If you are planning a new facade, a re-glaze or a green-rated fit-out, contact our team for a free measured quote and we will specify the right VLT for your building, budget and elevation.
