Choosing a window film involves two requirements that can pull in opposite directions: protecting occupant privacy and maintaining adequate natural light. Film opacity — measured by Visible Light Transmittance (VLT) — is the key parameter for calibrating this balance to each room's specific constraints.
Visible Light Transmittance (VLT): The Reference Metric
VLT expresses, as a percentage, the fraction of visible-spectrum light flux that passes through the glazing and film combined. A VLT of 75% means 75% of visible light is transmitted; conversely, a VLT near 1% characterises a fully blackout film. This parameter is measured under standardised protocols and is listed in the data sheet of each product reference.
It is important to distinguish VLT from the degree of frosting: two films may have comparable VLT values while producing very different diffusion effects. Opalescent frosted films diffuse light without blocking it entirely, blurring visibility in both directions. Externally reflective films exploit light asymmetry to ensure privacy exclusively from inside — as long as the outside is brighter than the interior.
Viewing Direction: The Overlooked Variable
A critical factor when selecting a frosted film: the frosting effect is not necessarily symmetrical.
- Opalescent frosted films (MAT LIGHT PET, MAT WHITE PET, MAT OPAL PET): these diffuse light bidirectionally. Privacy is assured in both directions, day and night, regardless of exterior light levels.
- Externally reflective films (Vista 90 C, Vista 90 XC): the mirror effect on the exterior side provides daytime privacy, but reverses as soon as the interior becomes brighter than the exterior — typically in the evening. These films are not a sufficient privacy solution at night without a supplement (blind, curtain).
This distinction is particularly important for spaces used in the evenings — bedrooms, bathrooms, offices whose glazing faces a public area.
Opacity Levels in the Solar Screen Discrétion Range
The Solar Screen Discrétion range covers four distinct opacity levels, characterised by their VLT measured on 3 mm clear glazing:
| Reference | VLT (%) | Visual Effect | Primary Use |
|---|---|---|---|
| MAT LIGHT PET | 75 | Very light opalescent veil | Maximum light, distance privacy |
| MAT FROST PVC | 70 | Standard frosted, etched look | General use, large surfaces |
| MAT WHITE PET | 40 | Marked frosting, silhouettes blurred | Enhanced privacy with residual light |
| MAT OPAL PET | 30 | High opacity, dimmed ambience | Reinforced privacy, sensitive rooms |
| WHITE OPAQUE | 1 | Full blackout, zero transmission | Complete visual isolation |
Source: Solar Screen technical data sheets, measurements on 3 mm clear single glazing.
Recommendations by Configuration
The right opacity level depends on three combined parameters: the degree of privacy required in the space, the desired natural light input, and the distance from the outside observer. For rooms requiring high privacy (bedrooms, bathrooms), a VLT of 30–40% ensures effective opacity while preserving some residual light diffusion. For communal living spaces or open-plan offices, a VLT of 70–75% is typically sufficient from a few metres away. Where complete visual isolation is needed — technical rooms, secure spaces — references with a VLT near 1% are the only appropriate solution.
Parameters to Check Before Installation
Regardless of the opacity level chosen, several factors determine the outcome:
Glazing type: on Low-E or laminated double glazing, the risk of thermal stress caused by film absorption must be assessed in advance. Solar Screen data sheets specify compatibility by glazing type.
Viewing distance: a film with a VLT of 70–75% may be sufficient when the nearest observer is more than 5–6 metres away, whereas glazing directly overlooking a pavement will require a higher-opacity film (VLT ≤ 40%).
Orientation and night-time use: a window heavily exposed in the evenings may call for an opalescent frosted film rather than a mirror film, whose effectiveness reverses once interior light levels exceed those outside.
For a deeper look at selection criteria by use type, the article Adhesive Privacy Films: Expert Advice covers the main scenarios for residential and professional environments — while the Solar Screen Discrétion range brings together all references with downloadable data sheets.