Light blocking and sheerness are two different textile properties that influence how fabrics perform, appear and are evaluated during product development and quality control.
Although both relate to how light interacts with a material, they describe different characteristics:
- Light blocking describes how much light passes through a fabric.
- Sheerness, sometimes described in terms of opacity, refers to how visible the background behind a fabric appears when viewed in reflected light.
Understanding the difference is important for textile manufacturers, brands, product developers and quality teams because both properties can directly affect product performance, customer satisfaction and approval decisions.
What Is Light Blocking?
Light blocking refers to a fabric’s ability to prevent light from passing through the material.
The higher the light-blocking performance, the less transmitted light reaches the opposite side of the fabric.
Light blocking is most commonly associated with:
- Blackout curtains
- Window coverings
- Blinds
- Drapery fabrics
- Light-control interior textiles
For manufacturers, light-blocking performance is often a technical product specification rather than simply a visual characteristic.
Why Light Blocking Matters
Effective light blocking can help support:
- Privacy
- Room darkening
- Glare reduction
- Energy efficiency
- UV protection
- Occupant comfort
Consumers increasingly expect claims such as “blackout,” “room darkening” or “light blocking” to deliver consistent performance. Manufacturers therefore need objective measurement methods to validate these claims and maintain uniformity between suppliers and production batches.
What Is Sheerness?
Sheerness describes how much of the background behind a fabric remains visible.
Unlike light blocking, sheerness is evaluated using reflected light rather than transmitted light.
A fabric may block a significant amount of light while still appearing visually sheer, depending on factors such as:
- Fabric construction
- Yarn density
- Fabric color
- Stretching
- Viewing conditions
- Background contrast
This distinction matters because consumers generally evaluate sheerness visually. A fabric may meet a light-blocking specification yet still appear too transparent during normal use.
Light Blocking vs. Sheerness: What Is the Difference?
| Property |
Light Blocking |
Sheerness |
| Measures |
Amount of transmitted light |
Visibility of the background through the fabric |
| Measurement Mode |
Transmission |
Reflectance |
| Common Applications |
Curtains, blinds and drapery |
Apparel, activewear and fashion textiles |
| Primary Concern |
Light control |
Visual opacity |
| Influenced By |
Fabric construction, thickness and density |
Color, stretching, background and lighting |
Although the terms are sometimes used interchangeably, they are not equivalent textile properties and should not be evaluated using the same measurement method.
How Is Light Blocking Measured?
Light blocking can only be measured using a spectrophotometer capable of operating in transmission mode.
During a transmission measurement:
- A light source directs light through the fabric.
- The instrument measures how much light reaches the detector.
- The result quantifies how effectively the material prevents light from passing through it.
Because this measurement is objective and repeatable, transmission testing helps manufacturers establish product specifications and maintain consistent performance across suppliers and production batches.
Industry Test Methods for Light-Blocking Fabrics
In 2014, the American Association of Textile Chemists and Colorists (AATCC) introduced two methods for evaluating the light-blocking performance of window coverings:
- AATCC TM148-2014: Photodetector Method
- AATCC TM203-2014: Spectrophotometric Method
Manufacturers using spectrophotometers with transmission capabilities can perform testing according to the AATCC TM203 method. Datacolor spectrophotometers with transmission measurement capabilities can support this type of evaluation.
The method is applicable to many types of fabrics, although some open or mesh constructions may require additional consideration.
Related reading: What Are the Different Types of Color Measurement Instruments? and Using a Spectrophotometer for Color Measurement.
How Is Sheerness Measured?
Sheerness is evaluated using reflectance measurements.
In this measurement method:
- The fabric sample is positioned in front of the instrument.
- Light reflects from the fabric and the background behind it.
- The instrument evaluates how the sample and background interact visually.
Because sheerness depends on visual appearance, several variables can influence the result:
- Fabric color
- Fabric construction
- Stretching
- Moisture content
- Surface texture
- Background color
Manufacturers should therefore establish standardized sample preparation, background and measurement conditions when evaluating opacity or sheerness.
Consistent procedures improve repeatability, reproducibility and quality control across teams and locations.
Related reading: Sample Measurement Technique in Digital Color Communication and Best Practices for Textile Sample Measurement.
Light-Blocking Applications: Window Coverings and Interior Textiles
The most common application for light-blocking measurement is window coverings.
Consumers often encounter product descriptions such as:
- Blackout curtains
- Room-darkening curtains
- Light-blocking blinds
- Insulated drapery systems
- Light-blocking curtain liners
Interestingly, while AATCC provides test methods for measuring light blocking, it does not define universal performance thresholds for marketing terms such as “light blocking” or “blackout.”
As a result, manufacturers and retailers often establish their own performance requirements. The amount of light reduction needed for a product to qualify as blackout may therefore vary between brands, retailers and product categories.
For quality managers and product developers, objective testing helps maintain consistency across suppliers and production batches, even when product specifications differ.
Why Consumers Choose Light-Blocking Window Coverings
Light-blocking products are used for more than simply making bedrooms darker.
Common applications and product benefits include:
- Reducing unwanted daylight in bedrooms
- Creating darker conditions in media rooms
- Improving privacy
- Reducing glare on screens
- Limiting solar heat gain
- Helping protect furniture and flooring from fading
- Supporting thermal comfort
Some manufacturers also market heavier or lined curtains as helping to reduce drafts or outside noise. However, these characteristics require separate testing methods and cannot be determined through spectrophotometric light-blocking measurements alone.
Sheerness Applications: Apparel and Fashion Textiles
Sheerness is particularly important in apparel and fashion applications.
Depending on the product and design intent, sheerness may be:
- A desirable design feature
- A quality concern
- A compliance risk
- A source of customer complaints
When Sheerness Is Desired
Fashion designers may intentionally select sheer or semi-transparent fabrics to create:
- Lightweight garments
- Layered looks
- Decorative effects
- Breathable constructions
- Luxury or delicate aesthetics
In these applications, sheerness is part of the intended visual design and should be measured consistently to preserve the desired appearance across different colors and production batches.
When Sheerness Becomes a Problem
Excessive sheerness can negatively affect customer perception and product acceptance in garments such as:
- Leggings
- Yoga pants
- Activewear
- Swimwear
- Uniforms
- Light-colored trousers
- Performance apparel
A fabric that appears opaque while relaxed may become noticeably transparent when stretched during normal wear.
This is not necessarily caused by a strong light source behind the garment. Instead, reflected light can reveal the contrasting background behind the material, such as the wearer’s skin or undergarments.
This makes real-world testing essential during product development.
Variables That Affect Fabric Sheerness
Sheerness is influenced by several interacting factors. Testing only one sample under one condition may not accurately represent how the final garment will perform.
Fabric Color
Light and dark colorways may show different levels of visual opacity, even when the fabric construction is identical.
Color contrast between the material and the background can strongly influence how sheer the fabric appears.
Stretching
Stretching increases the space between yarns and can make the background more visible.
For stretch garments, measurements should reflect realistic levels of extension rather than testing the fabric only in a relaxed state.
Background Color
The selected background influences perceived opacity. A dark textile over a light background may appear different from the same textile over a darker or skin-toned background.
Testing should therefore use standardized backgrounds that reflect the intended application.
Fabric Construction
Yarn size, knit or weave structure, fabric weight and density all influence how visible the background becomes.
Two fabrics with a similar color and weight may have very different opacity because of their construction.
Moisture
Some fabrics become more transparent when wet. This is particularly relevant for swimwear, sportswear and outdoor clothing.
Surface and Finishing Treatments
Coatings, brushing, calendaring and other finishing treatments can alter surface reflection and perceived sheerness.
Why Real-World Testing Is Essential
A complete sheerness evaluation should reflect how the consumer will actually use the product.
Depending on the application, this may require testing:
- Multiple fabric colors
- Different skin-tone or standardized backgrounds
- Relaxed and stretched samples
- Dry and wet conditions
- Different lighting conditions
- Finished garments as well as flat fabric samples
For example, a manufacturer evaluating activewear should not assume that a measurement taken from an unstretched fabric sample represents the appearance of the garment during movement.
Testing the material under realistic use conditions can help identify problems before production and reduce the risk of returns, complaints and reputational damage.
Why Textile Manufacturers Need Objective Measurement
Visual assessments alone are rarely sufficient for evaluating sheerness or light blocking.
Human perception varies because of factors such as:
- Lighting conditions
- Viewing angle
- Observer differences
- Background contrast
- Sample positioning
- Environmental conditions
Objective measurement provides:
- Repeatable results
- Clear supplier specifications
- Documented quality standards
- Reduced approval disputes
- Improved product consistency
- More reliable comparisons between batches
For global supply chains, measurable specifications help ensure that suppliers, laboratories and quality teams evaluate materials using the same criteria.
This reduces the risk of rejected materials, rework, delayed approvals and inconsistent product performance.
Related reading: Keys to Reliable Digital Color Communication and Why Should You Care About Inter-Instrument Agreement?.
Bringing Consistency to Textile Appearance Evaluation
Light blocking and sheerness are distinct textile properties that require different measurement approaches.
- Light blocking measures how much light passes through a material and is commonly evaluated for window coverings and interior textiles.
- Sheerness describes how visible a background appears through a fabric and is particularly important for apparel and fashion applications.
Because these properties describe different interactions between light and fabric, they should not be evaluated using the same instrument mode or testing procedure.
By using objective measurement methods and standardized testing conditions, textile manufacturers can improve quality control, support supplier alignment and make more consistent product decisions throughout development and production.
Practical Quality Control Recommendations
To create a reliable workflow for light-blocking and sheerness evaluation, manufacturers should define clear procedures for sample preparation, measurement and approval.
Define the Product Requirement
Establish whether the primary concern is transmitted light, visual opacity or both.
A blackout curtain and a pair of leggings may both involve light interaction, but they require different test methods and performance criteria.
Select the Correct Measurement Mode
- Use transmission mode to measure light blocking.
- Use reflectance mode to evaluate sheerness or opacity.
Using the wrong measurement method can produce results that do not represent actual product performance.
Standardize Sample Preparation
Document how samples should be prepared before measurement, including:
- Sample dimensions
- Number of fabric layers
- Sample orientation
- Conditioning requirements
- Stretch level
- Moisture condition
- Background color
Measure Multiple Areas
Textile materials may not be completely uniform. Measuring several locations can help identify variation caused by fabric construction, finishing or production inconsistency.
Test Relevant Colorways
Do not assume that every colorway will have the same opacity or light-blocking performance.
Light, dark and highly saturated colors may interact differently with the selected background and measurement conditions.
Include Realistic Use Conditions
Where relevant, evaluate samples under conditions that reflect actual product use, such as stretching, wetness, garment construction or installation in front of a window.
Define Measurable Tolerances
Set clear numerical limits or internal product specifications rather than relying on subjective terms alone.
This is especially important when marketing claims such as “blackout,” “room darkening” or “opaque” are used.
Align Suppliers and Laboratories
All locations should use the same measurement mode, sample preparation method, instrument settings and approval criteria.
This helps improve reproducibility and reduces disputes between brands, suppliers and testing laboratories.
How Datacolor Supports Textile Measurement
Datacolor helps textile manufacturers, brands and suppliers measure and manage color and appearance more consistently.
Relevant solutions include:
- Spectrophotometers with reflectance measurement capabilities
- Instruments that support transmission measurements
- Color quality control software
- Digital color standards and communication workflows
- Textile-focused color management solutions
- Professional support and workflow assessment services
Explore Datacolor’s Textile and Apparel Solutions, Datacolor Spectrophotometers and Color Management Software to build a more repeatable textile quality workflow.
Frequently Asked Questions About Light Blocking and Sheerness
Are light blocking and sheerness the same?
No. Light blocking measures how much light passes through a fabric, while sheerness describes how visible the background behind the material appears.
How is light blocking measured?
Light blocking is measured in transmission mode using a spectrophotometer capable of measuring the amount of light that passes through a fabric.
How is fabric sheerness measured?
Sheerness is evaluated using reflectance measurements, usually by comparing how the fabric appears against a defined background.
Can a fabric block light and still look sheer?
Yes. Light blocking and visual opacity are different properties. A fabric may reduce transmitted light but still reveal the background under reflected light.
What factors affect fabric sheerness?
Fabric color, construction, yarn density, stretching, moisture, surface texture, background color and lighting conditions can all influence perceived sheerness.
Why should stretch fabrics be tested while stretched?
Stretching increases the space between yarns and can make the background more visible. Testing only a relaxed sample may not represent how the finished garment performs during use.
What products commonly require light-blocking testing?
Light-blocking testing is most commonly used for curtains, blinds, drapery, window coverings and other interior textiles designed to control incoming light.
What products commonly require sheerness testing?
Sheerness testing is especially relevant for leggings, activewear, swimwear, uniforms, lightweight apparel and fashion textiles.
Is there one universal standard for blackout curtains?
No. While AATCC provides methods for measuring light blocking, manufacturers and retailers may define their own performance thresholds for terms such as “blackout” or “light blocking.”
Can visual inspection replace objective measurement?
No. Visual inspection remains useful, but objective instrument measurements provide more repeatable and comparable results across suppliers, batches and locations.
Improve Textile Appearance and Performance Evaluation
Objective measurement helps manufacturers distinguish between light blocking and sheerness, establish clearer specifications and reduce inconsistent product decisions across the supply chain.
Connect with a Datacolor Color Expert
Want to Get Color Right? 
