Attenuation equivalent testing, commonly expressed as the shade number, quantifies how effectively protective goggles filter intense optical radiation produced by welding arcs, furnaces, and other high-radiance sources. This technical article explains the principle of attenuation equivalent determination, the applicable EN, ISO, ANSI, and GB standards, the measured parameters, laboratory test methods, and typical industrial applications. It is intended for PPE manufacturers, safety engineers, procurement specialists, and quality assurance personnel who need to verify the protective performance of high-intensity light protective goggles.
Technical Overview
High-intensity light protective goggles are designed to shield the eyes from hazardous visible, ultraviolet (UV), and infrared (IR) radiation generated by processes such as arc welding, plasma cutting, furnace operation, and UV curing. The attenuation equivalent, most commonly expressed as the shade number (also called the scale number), is the primary index describing how strongly a filter attenuates luminous radiation. A higher shade number indicates stronger attenuation and greater protection against bright light sources.
According to EN 169 and equivalent provisions in ISO 16321 and GB/T 3609, the shade number N is derived from the luminous transmittance τv of the filter using the relationship:
- N = (7/3) × log10(1/τv)
- Each increment of one shade number reduces luminous transmittance by a factor of approximately 2.7
- A shade number of 10, for example, corresponds to a luminous transmittance of roughly 0.0013%
Attenuation equivalent verification is performed by accredited third-party laboratories using spectrophotometric measurement of spectral transmittance combined with weighted calculations defined in the relevant optical test standards. Compliance with these requirements is mandatory for personal protective equipment (PPE) conformity assessment in most major markets.
Testing Samples
Welding Goggles and Welding Filter Lenses
Passive welding filters made of mineral glass, polycarbonate, or coated substrates, supplied as complete goggles, replaceable filter windows, or cover plates. These are the most common samples submitted for shade number determination under EN 169 and GB/T 3609.1.
Furnace and Foundry Goggles
Eye protectors intended for hot-work environments such as metallurgical furnaces, glassmaking, and foundry operations. These samples are evaluated for infrared attenuation in accordance with EN 171 and related specifications.
UV Protective Eyewear
Goggles worn during UV curing, sterilization, photolithography, and arc-lamp work, assessed for solar-weighted ultraviolet transmittance in accordance with EN 170 and ANSI Z87.1 requirements.
Auto-Darkening Welding Filters
Electronic liquid-crystal filters are tested in both the light state and the dark state, and the dark-state attenuation equivalent must match the marked shade number. Switching behavior is typically covered under EN 379, while the attenuation values themselves are measured using the same optical methods.
Other High-Radiance Protective Devices
Gas-welding goggles, brazing and soldering eyewear, plasma-cutting protectors, and laser protective eyewear, the latter being evaluated under EN 207, EN 208, or ANSI Z136 series specifications depending on the application.
Testing Items
Depending on the product type and target standard, an attenuation equivalent test program typically includes the following parameters:
- Luminous transmittance (τv) measured over the visible spectrum of 380 nm to 780 nm
- Attenuation equivalent (shade number) calculation and verification against the marked value
- Solar-weighted ultraviolet transmittance (τSUV) covering the 200 nm to 400 nm region
- Near-infrared transmittance in the 780 nm to 1400 nm band, plus mid-infrared bands up to approximately 2600 nm where required
- Optical class assessment: spherical refractive power, astigmatic refractive power, and prismatic power differences
- Light diffusion (haze) of the filter material
- Transmittance uniformity across the area of vision
- Optional supplementary items such as impact resistance, ignition by hot particles, and corrosion resistance, which are non-optical tests covered by EN 168, ANSI Z87.1, or GB 14866
Testing Methods
Spectral Transmittance Measurement
A calibrated double-beam UV-Vis-NIR spectrophotometer is used to scan the filter across the wavelength range of approximately 200 nm to 2600 nm. Samples are conditioned at a standard laboratory atmosphere, measured against an air or reference-beam baseline, and scanned at multiple positions to confirm uniformity. Instrument wavelength accuracy, spectral bandwidth, and photometric accuracy are controlled and documented to ensure traceable results.
Weighted Transmittance Calculation
The measured spectral transmittance data are numerically integrated with weighting functions defined in the optical test standards:
- Luminous transmittance is calculated using the CIE photopic spectral luminous efficiency function V(λ) combined with CIE standard illuminant A
- Ultraviolet protection is evaluated by weighting spectral transmittance with the solar UV action spectrum applicable to the hazard band
- Infrared attenuation is assessed against the thermal-effect weighting function and the band-by-band maximum transmittance limits assigned to each shade number
Shade Number Determination and Tolerance Evaluation
The calculated luminous transmittance is converted to a shade number using the EN 169 logarithmic formula. The measured value must fall within the tolerance band defined for the marked scale number in the governing standard. Filters with dual or gradient shade markings are evaluated at each specified zone. Test reports state the measured transmittance, the computed shade number, the applicable tolerance, and a pass or fail conclusion.
Optical Quality Verification
Refractive power and prismatic differences are determined using the lensmeter or telescope-based methods described in EN 167 and equivalent ISO 16321-2 procedures. Light diffusion is measured to confirm that added filtration does not introduce unacceptable scatter, which would reduce visual clarity and safety.
Quality Control During Production
Manufacturers apply the same photometric principles in incoming-material inspection, in-process monitoring, and outgoing batch sampling, using production spectrophotometers correlated against reference laboratory measurements. Periodic type testing by accredited third-party laboratories supports ongoing conformity of the production samples with the originally certified design.
Applications
Attenuation equivalent testing supports safety and compliance across a wide range of industries and use scenarios:
- Welding and cutting: verification of shade numbers for MMA, MIG/MAG, TIG, submerged arc, oxy-fuel, and plasma processes, matched to welding current guidance in EN 169 and ANSI Z49.1
- Metallurgy and foundries: confirmation of IR attenuation for furnace tenders, pouring operators, and forge workers
- Glass, ceramics, and kiln operations: qualification of high-shade filters against continuous bright thermal radiation
- UV processing industries: validation of eyewear used near curing lamps, sterilization systems, and photolithography equipment
- Laser workplaces: attenuation verification for laser protective eyewear under EN 207 and EN 208 marking rules
- Regulatory conformity: technical documentation for PPE market approval, batch release, tender qualification, and workplace safety audits
- Research and development: characterization of new filter materials, coatings, and auto-darkening cells during product design
FAQ
What exactly is the attenuation equivalent of protective goggles?
The attenuation equivalent is a logarithmic index, the shade number, that expresses how much visible light a filter blocks. It is calculated from the luminous transmittance and is the principal marking on welding filters and other intense-light eye protectors.
How is the shade number different from optical density?
Both are logarithmic measures, but they use different scaling factors. Optical density is log10(1/τ), while the shade number equals 7/3 of optical density. A shade number of 12 therefore corresponds to an optical density of approximately 5.14.
Which shade number is appropriate for a given welding process?
Guidance tables in EN 169, ANSI Z49.1, and GB/T 3609.1 link shade numbers to process type and current. Typical examples include shade 9 to 11 for TIG welding below 100 A and shade 12 to 13 for high-current MIG or MMA welding. The final selection must also consider operator comfort and arc brightness.
Which standards govern attenuation equivalent testing?
The principal specifications are EN 166 and EN 167 for general and optical test requirements, EN 169 for welding filter shade numbers, EN 170 for UV filters, EN 171 for IR filters, EN 379 for auto-darkening filters, ISO 16321-1 and ISO 16321-2 for industrial eye protection, ANSI Z87.1 in North America, GB/T 3609.1 and GB 14866 in China, and JIS T 8141 in Japan.
What samples are required and how long does testing take?
Laboratories generally request several complete goggles or filter specimens in clean, unmarked condition. Spectrophotometric and optical testing is typically completed within a short working cycle, while full standard-compliance programs that include mechanical tests require additional time and sample quantities.
Are auto-darkening filters tested differently?
Their attenuation is measured in both the passive light state and the energized dark state. The light-state shade, dark-state shade, switching time, and delay time are all verified, because inadequate dark-state attenuation or slow switching can expose the wearer to arc flash.