One of the Illumination Engineering Society of South Africa (IESSA)’s core goals is to support and educate professionals in the lighting industry. Whether a designer, engineer, consultant, or policymaker, understanding the fundamentals of photometry is essential to your role in shaping better-lit environments.
In this article, IESSA takes a closer look at photometry, what it is, how it works, and the questions professionals often ask about it.
What is photometry?
Photometry is the measurement of the brightness of light as perceived by the human eye. Unlike radiometry, which measures all optical radiation (including ultraviolet and infrared), photometry focuses solely on visible light – how bright a light source appears to people.
This makes photometry a human-centric science, essential for applications such as architectural lighting design, street lighting, interior lighting planning, and compliance with lighting standards.
Frequently asked questions about photometry
Q1: What is photometry?
Photometry is the measurement of the brightness of light that the human eye can perceive. It considers how the eye responds to different wavelengths, emphasising visual perception rather than just energy output.
Q2: What is the basic unit of measurement used in photometry?
The lumen (lm) is the basic unit of photometric measurement. It quantifies the total amount of visible light emitted by a source per unit of time.
Q3: True or false: Photometry does not depend on the sensitivity of the eye to wavelength.
False. Photometry does depend on how sensitive the human eye is to different wavelengths. Our eyes are more sensitive to green light (~555 nm) and less sensitive to the red or violet ends of the spectrum, and photometric measurements are adjusted accordingly.
Q4: Does the human eye react differently to various wavelengths of visible light?
Yes. The human eye reacts with varying sensitivity across the visible spectrum. This is why two light sources with the same radiant energy can appear differently in brightness, because of how the eye perceives their wavelengths.
Q5: What is meant by radiometry?
Radiometry is the science of measuring optical radiation across a broader range than photometry, including ultraviolet, visible, and infrared light. Unlike photometry, it does not consider human visual response and is used for applications such as solar energy, laser safety, and thermal imaging.
Why photometry matters in lighting design
In a world increasingly driven by energy efficiency, environmental consciousness, and human-centric design, photometry is not just a technical concept; it’s a foundational tool for creating meaningful lighting solutions. Whether you're designing public infrastructure, commercial spaces, or residential environments, photometry plays a crucial role in guiding decisions.
Here's how a strong understanding of photometry benefits lighting professionals.
- Design lighting systems that meet both regulatory standards and human comfort needs: Lighting design is more than just installing luminaires; it's about ensuring compliance with national and international standards while also enhancing the experience of the people who use the space. Photometry helps designers assess illuminance levels, glare, and uniformity to meet regulatory guidelines, such as SANS or EN standards. At the same time, it ensures that lighting fosters visual comfort, safety, and productivity, adapting to human needs in workplaces, on roads, in schools, and in public spaces.
- Evaluate the performance of different lighting products accurately: Two products may consume the same wattage but deliver vastly different levels of visible brightness. Photometric data, such as luminous flux (lumens), luminous intensity (candela), and illuminance (lux) – helps professionals compare lighting products objectively. This ensures that specifications are based on visual performance, not just electrical consumption, allowing for smarter procurement and system design.
- Balance energy efficiency with human perception: Optimising for energy efficiency is vital, but it shouldn’t come at the cost of visual clarity or ambience. Photometry helps strike this balance by allowing professionals to design lighting that feels bright and clear to the eye, even at lower power levels. This is particularly important in LED design, where optical control and human response must be finely tuned for both comfort and efficiency.
- Inform intelligent lighting design and smart controls: As lighting systems evolve to become more responsive and adaptive, photometry supports innovations like dynamic lighting, daylight harvesting, and human-centric lighting. By understanding how light is perceived at different times of day and in different settings, designers can craft environments that are biologically attuned to human rhythms, improving wellbeing, sleep, and concentration.
- Support sustainability goals: Accurate photometric planning prevents over-lighting and reduces unnecessary energy consumption. It ensures that every lumen counts, helping cities, buildings, and homes reduce their carbon footprint while still delivering high-quality lighting experiences.
Whether you're specifying luminaires, writing tenders, evaluating products, or influencing urban lighting policy, photometry gives you the insight to make data-driven, human-centred decisions.