luox – A new open-access, open-source platform for spectral computations

July 29, 2021
Manuel Spitschan
Manuel Spitschan
luox – A new open-access, open-source platform for spectral computations

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Steps towards an ecosystem and culture of open-source tooling in light and lighting

By Manuel Spitschan

Melanopsin demands new metrology and new metrics

Around 20 years ago, a new type of cell was discovered in the human retina, the fine layer of tissue at the back of our eyes that enables us to see the world in its colourful detail. These cells, the intrinsically photosensitive retinal ganglion cells (ipRGCs), are sensitive to light independent of the canonical photoreceptors, the cones and the rods. The ipRGCs are a relatively rare type of retinal cell but very important for our physiology and behaviour: through the photopigment melanopsin, which is sensitive to short-wavelength light, the ipRGCs encode information about the time of day through light. These signals are sent to the hypothalamus, where they influence our circadian rhythms the production of the hormone melatonin.

Since then, this fundamental discovery has changed how we think about light, from the measurement of light itself to the importance of considering the circadian and neuroendocrine effects on people in lighting design. We know how that the best way to describe these effects is by considering the activation of the melanopsin component of ipRGC signals (Brown et al., 2020). While the cones and rods can also influence the circadian and neuroendocrine pathways, this may only be the case in specific regimes. Importantly, research on the non-visual effects of light is evolving and developing quickly.

In 2018, the CIE published a new international standard for the quantification of light in relation to the ipRGCs. Consistent with other CIE quantities – including the CIE 2006 physiologically relevant cone fundamentals based on the work of Stockman, Sharpe and colleagues, and the V’(λ) curve for scotopic luminosity – the new CIE S 026 standard proposes a set of spectral sensitivity curves. In addition, it introduces the α-opic equivalent daylight illuminance (or luminance) of a spectrum, which gives the illuminance in lux of a daylight spectrum (D65) that would produce the same α-opic irradiance (or radiance) as the spectrum in question. Even as we are learning knew information on how the cones and rods may influence neuroendocrine and circadian physiology, the new CIE system provides a theory-neutral system of quantifying the effect of light on people integratively.

Standards and guidelines need tooling

Science works by drawing from past research. However, aggregating information across studies from different laboratories and research groups requires a common language to describe at least the stimulus conditions used in a given experiment. To this end, my colleagues and I wrote guidelines for reporting light exposure in chronobiology and sleep research experiments (Spitschan et al., 2019). These guidelines propose at a minimum to provide spectral irradiance in tabulated form, along with other descriptors. The guidelines serve two purposes: 1) improving the reporting of lighting conditions in the field so that research could in principle be reproduced and replicated; and 2) facilitating later efforts to aggregate data in meta-analyses and other aggregation efforts. In our checklist, we ask researchers to:

  • Measure and report the spectral power distribution of the acute stimulus from the observer’s point of view at a known and specified angle and distance from the source
  • Measure and report the spectral power distribution of the background light environment from the observer’s point of view at a known and specified angle and distance from the source
  • Make spectra available in tabulated form
  • Report α-opic irradiances (or radiances) and illuminance
  • Describe the timing properties of the stimulus (clock time, duration, and pattern)
  • Describe the spatial properties of the stimulus (spatial arrangement and extent)
  • Report measurement conditions and equipment

The CIE recently published a timely guide for reporting information about research studies on the non-visual effects that takes a more generalist stance (CIE, 2020), making recommendations regarding other aspects of the study protocol to be reported. While standards and guidelines are essential, they are only one part of harmonising efforts. With the user in mind, it is important to provide tools that make it easy for users to conform to specific standards.

To this end, we developed luox, software that enables the processing of relative or absolute spectral power distributions (either radiance or irradiance) and calculates quantities based on these spectra that are displayed in the browser. Notably, no data are ever uploaded to a server, as the calculations are performed directly in the browser on the user’s computer. In addition to performing the calculations, luox facilitates the sharing of spectra using a package developed by Michael Herf called spdurl, which compresses the spectrum into the length of the URL. That is, the URL itself encodes the spectrum, requiring no additional input.

By design, luox is open-access and free to use. It is also open-source, and we have released the source code under the GPL-3.0 license. Further development of the platform is planned, including colour rendition metrics, such as ANSI/IES TM-30-20 and CIE 224:2017, and functionality to support import of spectral data in ANSI/IES TM-27-20 format. User contributions are also welcome, following our contributor policy. As luox is available as open-source software and does not rely on a server backend to run (it is deployed on a cloud platform), users could run their own luox instances, or make modifications to the code (subject to the GPL-3.0 license conditions). Importantly, luox was recently validated and endorsed by the CIE.

Toward (an) open and transparent research (culture)

Standards and guidelines and tooling, such as luox, will eventually help make research robust, reproducible, and, therefore, better and more trustworthy. It will also avoid “research waste,” i.e., replicating research that was not documented well or not reported at all. But ultimately, it is people who do research, and a lot of how we do science is very much a function of research culture. In psychology and human neuroscience, principles of reproducible research have recently become a focus of attention. Researchers can use several practical elements to make their research more reproducible – including making data, code, and materials routinely available as part of publications, and pre-registering hypotheses and analytic plans that test these hypotheses. When it comes to data, the FAIR guiding principles are of particular importance: Data should be findable, accessible, interoperable, and reusable to be useful (Wilkinson et al., 2016).

At no point in time has there been so much research activity on the non-visual effects of light in science. To inform policy, research on the non-visual effects of light must be robust. Part of this involves pulling at one side of the rope and ensuring that our science is reproducible, starting at describing the stimulus conditions. The future of lighting is open – let us future-proof it.

Tell us how we can improve luox by taking our survey. Completing the survey should take approximately 6 minutes. Please email Manuel Spitschan at with any questions.

Brown, T. M. (2020). Melanopic illuminance defines the magnitude of human circadian light responses under a wide range of conditions. J Pineal Res, 69(1), e12655. doi:10.1111/jpi.12655.

CIE. (2020). CIE TN 011:2020: What to document and report in studies of ipRGC-influenced responses to light. Vienna: CIE Central Bureau.

CIE. (2018). CIE S 026/E:2018: CIE System for Metrology of Optical Radiation for ipRGC-Influenced Responses to Light. Vienna: CIE Central Bureau.

CIE. (2006). CIE 170-1:2006: Fundamental chromaticity diagram with physiological axes – Part 1. Vienna: CIE Central Bureau.

Munafo, M. R., Nosek, B. A., Bishop, D. V. M., Button, K. S., Chambers, C. D., du Sert, N. P., . . . Ioannidis, J. P. A. (2017). A manifesto for reproducible science. Nat Hum Behav, 1, 0021. doi:10.1038/s41562-016-0021

Spitschan, M., Stefani, O., Blattner, P., Gronfier, C., Lockley, S. W., & Lucas, R. J. (2019). How to report light exposure in human chronobiology and sleep research experiments. Clocks Sleep, 1(3), 280-289. doi:10.3390/clockssleep1030024

Spitschan, M., Mead, J., Roos, C., Lowis, C., Griffiths, B., Mucur, P., & Herf, M. (2021). luox: novel open-access and open-source web platform for calculating and sharing physiologically relevant quantities for light and lighting. Wellcome Open Res, 6, 69. doi:10.12688/wellcomeopenres.16595.1

Spitschan, M. (2021). luox: Platform for calculating quantities related to light and lighting [Source code]. Available from (archived at

Wilkinson, M. D., Dumontier, M., Aalbersberg, I. J., Appleton, G., Axton, M., Baak, A., . . . Mons, B. (2016). The FAIR Guiding Principles for scientific data management and stewardship. Sci Data, 3, 160018. doi:10.1038/sdata.2016.18

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