Fabric Performance: Openness, Opacity and Light Transmission

The Key Numbers

When specifying a roller blind fabric, three numbers determine performance: openness factor, light transmission, and g-value. They are related but distinct, and each answers a different question.

Openness Factor

Openness factor (OF) is the percentage of the fabric’s total area that is open space. A 3% openness fabric has 3% of its area as holes; a 10% fabric has larger or more numerous openings. The range used in practice is typically 0% (blackout) to 14% (very open mesh).

Openness factor does not tell the whole story: a 3% openness white fabric will transmit very different amounts of light and heat to a 3% openness dark grey fabric.

Light Transmission (Tv)

Light transmission is the percentage of visible light that passes through the closed blind, measured using EN 14501 and reported as Tv. A dark fabric with 3% openness might have Tv of 0.03, while a white fabric at the same openness might have Tv of 0.10, because the white yarns themselves transmit more light.

Solar Factor (g-value)

As covered in our Solar Heat Gain guide, the g-value measures how much of the total solar energy passes through the fabric system. A reflective white fabric at 3% openness will have a considerably better g-value than a dark 3% openness fabric, because the white surface reflects more radiation back out through the glass.

Fabric Type	Openness	Typical Tv	Typical g-value	Best Application
Blackout	0%	0.00	0.05–0.15	Bedrooms, cinemas, hospital wards
Screen — dense (1%)	1%	0.01–0.03	0.10–0.20	South-facing offices, strong glare reduction
Screen — standard (3%)	3%	0.03–0.08	0.15–0.30	Most residential and commercial applications
Screen — open (5%)	5%	0.05–0.12	0.20–0.35	Views and daylighting priority
Screen — mesh (10%)	10%	0.08–0.18	0.25–0.40	Glare only, no heat reduction priority
Dim-out lining	0%	0.01–0.05	0.20–0.30	Curtain lining where some light welcome

Colour and Performance

The relationship between colour and performance is often misunderstood:

• White and light fabrics reflect solar radiation back through the glass. They have better g-values for solar heat reduction.
• Dark fabrics absorb solar radiation and re-radiate it as heat. They may reduce glare effectively while allowing significant heat gain.
• Metallic/reflective backings give the best solar heat performance at any colour, because the coating reflects radiation regardless of the front face colour.

For Part O compliance specifications, we always recommend fabrics with reflective backings on south and west elevations, regardless of the front face colour chosen for interior aesthetics.

The View-Through Trade-off

A common client question: “Can I still see out when the blind is down?” The answer depends on the relationship between interior and exterior light levels, not just the fabric openness. During the day, if external light levels are higher than interior levels, you can see out through a screen fabric. At night, if interior lights are on and external levels are low, anyone outside can see in clearly.

Blackout Fabrics

True blackout (0% light transmission) is achieved through layered constructions: typically a base fabric, one or more foam or acrylic blackout coatings, and a finishing face. EN 13120 grades blinds by their light exclusion from Class 0 (sheer) to Class 3 (full blackout). Note that even Class 3 fabrics allow light around the edges of an uncovered window reveal. For genuine blackout, a side-channel cassette system is required.

Curtain Linings

Curtain fabric performance is primarily governed by the lining choice, not the face fabric. Lining weights:

• Standard lining: Protects the face fabric from sun damage and adds body. Minimal thermal or light performance.
• Blackout lining: Foam or acrylic-coated. Full light exclusion, some acoustic benefit.
• Thermal lining (bump): Thick interlining between face fabric and lining. Significant improvement in U-value and acoustic performance. Standard specification for formal curtains in cold rooms.
• Interlining: Adds weight and drape; combined with blackout lining gives the best all-round performance.