Why Do Curtains Glow Red? The Science Behind Light Bleed Through Blackout Curtains
One of the most common questions we hear from customers goes something like this:
"You said your curtains are blackout. But at night, I can see red light coming through them. Does this mean the quality is bad? Are you misleading me?"
Today, we're setting the record straight. If your curtains glow red, it actually means they are doing their job well.
The Science of Light: Why Red Gets Through
Sunlight is composed of seven color bands: red, orange, yellow, green, cyan, blue, and violet. Each color has a different wavelength.
Red light has the longest wavelength (approximately 620-750nm) and therefore the strongest penetrating power. When light passes through curtain fabric, shorter wavelengths like blue, violet, and green are absorbed or blocked by the material. But red light, with its superior penetration ability, is the hardest to stop.
So if your curtains are glowing red at night, it means they have successfully blocked most other colors of light. Only the most stubborn red wavelengths are making it through.
The Light Test: What Your Curtains Are Telling You
Try this simple test at home. Take a flashlight or your phone light and press it against the back of your curtain in a dark room.
|
Light Blocking Level
|
Glow Color
|
What It Means
|
|---|---|---|
|
None
|
White light
|
Like sheer curtains, all light passes through freely
|
|
Low
|
Yellow/orange glow
|
Some colors are blocked, but significant light still penetrates
|
|
Good
|
Red glow
|
Most light is blocked. Only the strongest wavelength (red) remains
|
What this tells you:
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White glow = Your curtain has minimal to no light blocking capability
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Yellow glow = Your curtain offers basic light reduction but is far from blackout
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Red glow = Your curtain is performing at a high level, blocking nearly all visible light
This is why you'll often notice hotel curtains glowing red from the outside. That red glow is actually a sign of quality blackout performance.
Other Factors That Affect Light Bleed
Beyond the blackout rating, several factors influence what color light comes through:
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Fabric Color: Darker fabrics absorb more light overall, making red glow more prominent
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Fabric Density: Higher thread count and tighter weaves naturally block more light
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Lining Technology: Back coatings and black interlinings significantly change light transmission behavior
Three Ways to Eliminate Red Glow (If You Want To)
Not everyone minds a subtle red glow. But if you want total darkness with zero light bleed, here are three proven solutions:
Option 1: Add a Liner (Most Affordable)
When ordering custom curtains, request an additional liner layer:
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205gsm Liner: Basic light improvement with good value
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240gsm Liner: Higher density for better blackout, improved drape, and thermal insulation
A liner not only reduces light bleed but also improves the curtain's hang and adds year-round insulation.
Option 2: Upgrade to High-Performance Fabric (Best Results)
These fabrics have been tested and proven to deliver superior blackout performance:
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Windsor Jacquard: Woven jacquard with excellent light blocking
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Seren Chenille: Soft chenille texture with strong blackout properties
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Simon Chenille: Another chenille option with consistent, reliable performance
These fabrics typically achieve 90%+ light blocking, significantly reducing or eliminating red glow.
Option 3: Install a Dual Layer System (Best for Versatility)
Use a double rod or double track system:
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Outer Layer (near window): Hang sheer or semi-sheer curtains
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Leave these closed during the day for soft, filtered light
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Protects furniture and flooring from direct UV exposure
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Inner Layer (near room): Hang full blackout curtains
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Close at night for complete darkness
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Ensures optimal sleep quality
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This combination solves the light bleed issue while adding visual depth and flexibility to your window treatment.
The Bottom Line
Red light glow is not a sign of poor quality. In fact, it is evidence that your curtains are performing at a high level, blocking most of the visible light spectrum. The curtains you should truly worry about are the ones that glow white or yellow.
Next time a customer asks about red light bleed, share this explanation. It turns a potential complaint into a moment of education and trust building. And if they want total elimination of all light, you now have three clear solutions to offer.
Have questions about curtain blackout performance? Drop us a message or leave a comment below.
