Your Questions Answered: What Is HDRS Testing and Why Does It Matter?
Hybrid Diffuse Reflectance Spectroscopy (HDRS) is one of the most significant advances in sunscreen testing in decades, and it’s already approved for product labeling in Europe. Here, our team answers the most common questions we hear from formulators, brands, and regulatory teams.
What is HDRS, and what does the acronym stand for?
HDRS stands for Hybrid Diffuse Reflectance Spectroscopy. It is a sunscreen testing method developed and validated by ISO (published as ISO 23698) that uses light reflected off the skin, combined with an in vitro component, to measure SPF, UVA protection factor, and critical wavelength. “Hybrid” refers to the fact that the method combines an in vivo component (measurements taken on human subjects) with an in vitro component (a standard spectral radiometry measurement).
Is HDRS a brand-new technology, or has it been around for a while?
HDRS is new as a compendial method, but the underlying science goes back more than 20 years. Diffuse Reflectance Spectroscopy (DRS) was originally developed as a non-invasive research tool for examining chemical properties within the skin, such as oxygenated hemoglobin, collagen, melanin, and hydration. Researchers recognized its potential for sunscreen assessment roughly two decades ago, and a group of scientists, including the late Nick Kollias at Massachusetts General Hospital and Johnson & Johnson, Eduardo Rivolo, Kurt Cole, and Matthias Rohrer, eventually organized into what became known as the Alt SPF Consortium. Working with Solar Light Company, they spent years refining the technology and conducting the studies required for ISO validation. The method was formally published as a compendial standard in 2024.
How does HDRS actually work?
At a high level, the method works in two connected phases:
In the in vivo phase, a trained technician takes nine baseline reflectance measurements on each test site on a subject’s back (on either side of the midline, below the shoulder blades). Sunscreen is then applied and left to dry for at least 15 minutes, after which nine post-application measurements are taken. The device’s built-in software uses this data to calculate a pre-irradiation dose for the in vitro phase.
In the in vitro phase, the product is run through a spectroradiometer (per ISO 24443, the critical wavelength method) both with and without pre-irradiation.
All of the data is then compiled in a dedicated software program that levels the absorption curves, performs curve fitting, and generates a final report that provides SPF, UVA protection factor, and critical wavelength, everything needed for a broad-spectrum claim, in a single test run.
The in vivo component is particularly important for UVA: researchers found that human reflectance spectroscopy is significantly better at assessing UVA protection than UVB protection, which is why the hybrid approach is structured the way it is.
How does HDRS compare to traditional SPF testing methods like ISO 24444 or the FDA method?
The differences are meaningful in several ways:
Speed
A 10-subject HDRS panel can be completed in one to two days. Traditional SPF testing requires pre-qualifying subjects by determining their individual UV sensitivity (which involves UV exposure one day and readings the next), followed by iterative dosing sessions. From start to finish, a traditional panel realistically takes two to four weeks.
Scope
A single HDRS test generates SPF, UVA protection factor, and critical wavelength. With traditional methods, depending on your target market, you may need to run up to three separate studies to obtain the same information.
Subject experience
Traditional SPF testing requires inducing controlled erythema, a minor burn, in every test subject. HDRS applies a very mild UV dose that does not cause redness or inflammation. The EU has long prioritized methods that minimize harm to test subjects, and this was a significant motivating factor in the development and adoption of HDRS.
Data consistency
HDRS has built-in quality controls that traditional methods lack. The device measures the evenness of product application in real time. If the application does not meet a strict set of criteria, the analysis cannot proceed, removing one of the most significant sources of variability in sunscreen testing.
Correlation
Validation data from the Alt SPF Consortium shows a strong correlation between HDRS results and those generated by traditional ISO and FDA methods across the full SPF range.
What is the difference between HDRS and the “two-plate method”?
Both are DRS-based methods published by ISO in 2024. The two-plate method (ISO 23675) is a fully in vitro approach that uses two plastic plates, one smooth, one sandblasted, and is approved in the EU for SPF labeling only. It does not generate a UVA protection factor or support a broad spectrum claim.
HDRS (ISO 23698), by contrast, generates SPF, UVA protection factor, and critical wavelength in a single test, making it the more comprehensive option for brands that need a complete efficacy profile.
What equipment is required for HDRS testing?
The method requires a commercially available reflectance spectroscope called the Poly 602®, manufactured by Solar Light Company. This device is specifically designed and approved by ISO for use with the HDRS methodology. Solar Light also makes a monochromatic version (the Mono 602) that uses a single wavelength and is primarily a research tool; the Poly 602® uses multiple wavelengths and is the instrument approved for labeling purposes.
Where is HDRS currently approved for product labeling?
As of now, HDRS is fully approved in the European Union for labeling sunscreens with SPF, UVA protection factor, and broad spectrum claims. This is a significant development given that the EU represents one of the largest and most regulated sunscreen markets in the world.
Australia’s Therapeutic Goods Administration (TGA) is actively considering both HDRS and the two-plate method, either as a primary labeling method or as a tool for augmenting existing SPF claims. Given that many countries in Asia, South America, and elsewhere tend to follow European regulatory decisions, broader global adoption is a reasonable expectation.
HDRS is not currently approved for labeling in the United States, where the FDA method governs. That said, the US regulatory landscape for sunscreens continues to evolve, with Congressional acts and proposed rules that are gradually pushing the agency toward greater openness to alternative approaches.
How much does HDRS testing cost compared to traditional methods?
Roughly speaking, a full HDRS test, which delivers SPF, UVA protection factor, and critical wavelength, costs approximately the same as a traditional SPF test alone. That means you are getting two or three studies’ worth of information for the price of one, representing cost savings of up to 50% compared to running all three tests separately using traditional methods. Specific pricing varies by laboratory, but the efficiency gains are consistent.
Can HDRS be used during formula development, not just for final labeling?
Yes, and this is one of the more exciting practical applications. Because a panel can be completed in one to two days, HDRS can realistically be incorporated into the formulation process itself, something that is simply not feasible with traditional methods.
Potential applications during development include:
- Rapid screening across a portfolio of formulations to understand how ingredient or concentration changes affect SPF and UVA coverage
- Bridging studies when a minor formula change has been made, and you want to confirm the SPF has not been adversely affected, potentially without running a full traditional study (subject to regulatory and legal review)
- Design of experiments to systematically test a range of sunscreen actives or concentrations and generate comparative efficacy data quickly
Can HDRS data be used to support a product defense if an SPF claim is challenged?
Yes. Given the ongoing global scrutiny of SPF labeling, including product recalls in Australia, enforcement actions in Canada, and class action litigation in the United States, having additional, independently generated efficacy data is a meaningful risk-management measure. Running an HDRS test alongside or prior to market launch demonstrates due diligence and provides a second source of evidence that your product performs as labeled.
Is CPT Labs currently offering HDRS testing?
Yes! CPT Labs has an HDRS system in operation. We welcome clients who want to learn more about the method, observe it in action, or discuss how it fits into their testing and development strategy. Feel free to reach out to schedule a lab visit or to start a conversation about your specific needs.
Questions about HDRS or other sunscreen testing methods? Learn more about HDRS testing and contact our team directly. We’re happy to walk you through your options.