Essential Lighting Terms Explained: Technical Glossary

Table of Contents

Intro

Part 1: The Physics of Light & Vision

Part 2: Photometrics: Quantifying Light

Part 3: Color Science & Quality

Part 4: Optical Distribution & Glare Control

Part 5: Electrical & Mechanical Terms

Part 6: The Complete A-Z Lighting Glossary

Conclusion

Intro

Have you ever looked at an LED specification sheet and felt like you were reading a different language? You are not alone.

From "L70" to "MacAdam Ellipse," the lighting industry is filled with complex acronyms and technical jargon. However, understanding these lighting terms is no longer optional.

Whether you are an engineer, a lighting designer, or a procurement manager, mastering this terminology is critical. It ensures you aren't just buying "bright lights," but are selecting products that deliver energy efficiency, visual comfort, and long-term reliability for your projects.

This guide goes beyond a simple dictionary. We have organized the most essential concepts into a logical structure to help you make better decisions:

l The Physics of Light: Understanding the spectrum and human vision.

l Photometrics: How we measure and quantify light (Lumens, Lux, etc.).

l Color Science: Mastering CCT, CRI, and TM-30.

l Electrical & Mechanics: Drivers, dimming, and protection ratings.

l The A-Z Glossary: A quick-reference index for everything else.

Part 1: The Physics of Light & Vision

Before we discuss lamp specifications, we must understand the nature of light itself and how our eyes perceive it.

01. Light

Physically speaking, light is a form of electromagnetic radiation energy. It travels in waves and moves in straight lines from a source. While light exists throughout the universe, what matters in our industry is the specific band of energy that the human eye can detect.

Reference: https://en.wikipedia.org/wiki/Light 

02. Spectrum

The "Visible Spectrum" is the segment of the electromagnetic spectrum that the human eye can perceive. It ranges in wavelength roughly from 380nm (Violet) to 780nm (Red).

Why it matters: Understanding the spectrum is the foundation for understanding Color Rendering (CRI) and plant growth lighting (PAR).

03. Human Vision (Photopic, Scotopic, Mesopic)

Not all light can be perceived in the same way. Our eyes use two types of photoreceptors: Cones (for bright light and color) and Rods (for dim light and movement). This leads to three distinct vision states:

Photopic Vision (Day Vision):

l Occurs in bright conditions (>3 cd/m²).

l Cones are active. We see color and detail clearly.

l Peak sensitivity is at 555nm (Green-Yellow). Most standard indoor lighting is designed for this state.

Scotopic Vision (Night Vision):

l Occurs in very dark conditions (<0.001 cd/m²).

l Rods are active. We see in black and white (greyscale).

l Peak sensitivity shifts to 507nm (Blue-Green).

Mesopic Vision (Intermediate Vision):

l The Crucial Zone: This happens between light and dark (like twilight or street lighting). Both Cones and Rods are active.

l Application: In Street Lighting design, we must account for Mesopic vision. A cooler white light (which stimulates rods better) often improves visibility for drivers at night compared to warm yellow light, even if the lumen output is the same.

Part 2: Photometrics: Quantifying Light

One of the biggest confusions in lighting terms is the word "Brightness." In engineering, we don't use that word. Instead, we use four precise metrics.

04. Luminous Flux (Lumens - lm)

"How much light is coming out?"
Luminous Flux is the total amount of visible light emitted by a source in all directions. It is the raw "power" of the lamp.

Note: Higher lumens do not always mean "brighter" on your desk—it depends on where that light goes.

05. Luminous Intensity (Candela - cd)

"How strong is the beam in one direction?"
This measures the force of light in a specific direction.

Example: A laser pointer has low Lumens (total output) but extremely high Candela (intensity) because the light is focused. A spotlight will have higher Candela than a generic light bulb of the same wattage.

06. Illuminance (Lux - lx & Foot-candles - fc)

"How much light hits the surface?"
This is the most important metric for project acceptance. It measures the luminous flux falling on a specific unit area (like a desk or floor).

Ø Lux (lx): Lumens per square meter (Metric system, used globally).

Ø Foot-candle (fc): Lumens per square foot (Imperial system, used in the USA).

Conversion Formula:

1 Foot-candle ≈ 10.76 Lux
(Rule of thumb: 1 fc is roughly 10 Lux)

07. Luminance (cd/m² or Nits)

"How bright does it look to the eye?"
Illuminance is the light hitting the wall; Luminance is the light reflecting off the wall and entering your eye. This is the only photometric value we actually see.

08. Luminous Efficacy (lm/W)

"How efficient is the conversion?"

This measures how well a light source converts electricity (Watts) into visible light (Lumens). It is the MPG (Miles Per Gallon) of the lighting world.

09. Uniformity (U0)

Uniformity is the ratio of the minimum illuminance to the average illuminance on a surface (Emin / Eavg).

Why it matters: If a room has bright spots and dark spots (low uniformity), our eyes constantly have to adjust, causing visual fatigue. A uniformity ratio close to 1.0 is ideal but difficult to achieve; >0.4 is standard for most workspaces.

10. Reflectance

This is the percentage of light that a surface reflects rather than absorbs.

Design Tip: You can use the same lights in two different rooms, but if Room A has black walls (Low Reflectance) and Room B has white walls (High Reflectance), Room B will appear significantly brighter. Lighting design must always consider the material of the space.

Part 3: Color Science & Quality

Most manufacturers stop at CRI. But for high-end projects—museums, retail, and luxury hospitality—standard metrics are no longer enough. Here is the deep dive into modern color quality.

11. CCT (Color Temperature) & The Kruithof Curve

CCT measures the "warmth" or "coolness" of light, measured in Kelvin (K). But specifying CCT isn't just about picking a number; it's about psychology and comfort.

This brings us to the Kruithof Curve. This principle explains that visual comfort depends on the specific combination of brightness (Lux) and color temperature (CCT).

The Principle: Dim lighting looks natural when it's warm (think of a candle or sunset). Bright lighting looks natural when it's cool (think of a sunny blue sky).

The Trap: If you use a high color temperature (e.g., 6000K) at low brightness levels, the space will feel "eerie" or "cold," like a cloudy winter day. Conversely, high brightness with very warm light (e.g., 2700K) can feel intensely hot and unnatural.

Reference: https://en.wikipedia.org/wiki/Kruithof_curve 

12. CRI vs. TM-30 (Beyond Standard Color Rendering)

For decades, CRI (Ra) was the standard. It calculates the average score of 8 pastel colors (R1-R8).

Ø The Flaw of CRI: It ignores R9 (Saturated Red). A light source can score CRI 80 but fail miserably at rendering red tones (making skin look sickly or meat look brown). For quality projects, always demand R9 > 50.

Enter TM-30-15: The New High-Definition Standard
TM-30 is the modern, comprehensive system developed by the IES. It evaluates 99 color samples (instead of CRI's 8) and provides two critical metrics:

1. Rf (Fidelity Index): Similar to CRI, it measures how close the color is to natural light. (Scale 0-100).

2. Rg (Gamut Index): This measures saturation.

Rg = 100: Normal saturation.

Rg > 100: Colors look more vivid (oversaturated).

Rg < 100: Colors look dull (desaturated).

How to Read the Color Vector Graphic:

When looking at a TM-30 report, you will see a circle.

l Black Circle: Represents the reference source (Sunlight).

l Red Shape: Represents your LED light.

l Interpretation: If the red line pushes outside the black circle, that specific color (e.g., red or blue) will appear more vivid. If it shrinks inside, the color will look dull. This visual tool is essential for retail lighting design where "pop" is required.

Reference: https://en.wikipedia.org/wiki/Color_rendering_index 

13. SDCM & Binning (MacAdam Ellipse)

Have you ever installed 10 downlights, and noticed that two of them look slightly pinkish or greener than the others? This is a Binning issue.

We measure color consistency using the MacAdam Ellipse (or SDCM - Standard Deviation Colour Matching).

l The Concept: It defines a zone on the color chart where the human eye cannot distinguish the difference between two colors.

l 3-Step MacAdam: The gold standard for architectural lighting. The color difference is virtually invisible to the human eye.

l 5-Step MacAdam: Acceptable for general spaces (like warehouses), but noticeable if you look closely.

l 7-Step MacAdam: Cheap LEDs. Visible color variations are guaranteed.

Part 4: Optical Distribution & Glare Control

A great chip is useless without a great lens. This section covers how we control light to ensure it lands where we want it—without hurting our eyes.

14. Glare (UGR)

Glare isn't just "too much light"; it's light entering the eye at the wrong angle, causing discomfort or disability. We quantify this using UGR (Unified Glare Rating). Lower numbers mean better visual comfort.

Standard UGR Thresholds:

15. Shielding Angle (Cut-off Angle)

This is a physical attribute of the fixture design. It is the angle measured from the horizon down to the point where the light source first becomes visible.

Why it matters: A "Deep Baffle" design increases the shielding angle (e.g., >30°). This hides the bright LED chip from view, ensuring that you see the light effect on the object, not the light source itself. This is the hallmark of luxury lighting.

16. Beam Angle vs. Field Angle

These two are often confused, leading to messy lighting designs.

l Beam Angle (FWHM): The angle where light intensity drops to 50% of the center max brightness. This is your "primary" cone of light.

l Field Angle: The angle where light intensity drops to 10%.

l The Trap: A fixture might have a narrow 20° Beam Angle but a wide 60° Field Angle. This creates a "soft edge" or spill light that might unintentionally illuminate surrounding areas. For precise accent lighting, you want the Beam and Field angles to be relatively close.

17. Light Pollution (Dark Sky & ULR)

For outdoor projects, controlling where light doesn't go is as important as where it does go.

l ULR (Upward Light Ratio): The percentage of light emitted above the horizontal plane (skyward).

l Dark Sky Compliant: Fixtures designed with ULR = 0%. They eliminate "Sky Glow," protecting wildlife and our view of the stars. In many modern cities, this certification is mandatory for planning permission.

Part 5: Electrical & Mechanical Terms

The longevity of an LED fixture rarely depends on the LED chip itself. It depends on the electronics and the build quality. Here is what to check on the spec sheet.

18. LED Driver (Constant Current vs. Voltage)

The driver is the heart of the LED system.

l Constant Current (CC): The driver regulates the amperage (e.g., 350mA, 700mA). Used for most downlights, track lights, and high bays. It offers better efficiency and lifespan control.

l Constant Voltage (CV): The driver provides a fixed voltage (e.g., 12V, 24V). Primarily used for LED Strips and parallel circuits where the load length varies.

19. Power Factor (PF)

PF measures how effectively a device uses electricity, ranging from 0 to 1.

l The Benchmark: A PF > 0.9 is the industry standard for commercial lighting.

l The Reality: Cheap drivers often have a PF of 0.5. While this might not change the wattage bill for a home user, in a large commercial building, low PF causes "dirty power," stressing the electrical grid and potentially incurring utility penalties.

20. Dimming Protocols

Dimmers also come in different types.

l 0-10V: The analog standard for commercial offices. Simple and reliable.