Characteristics of Light

Shopping for light bulbs gets a lot easier once you know the three numbers that actually describe light: lumens (how bright), color temperature (how warm or cool), and CRI (how true colors look). Here's what each one means.

Lumens — How Bright Is It?

In the past, consumers used wattage to rate the brightness of a lamp. However, wattage is not an accurate indicator of brightness — it's simply a measure of the amount of energy consumed. Energy-efficient light sources such as LEDs and Compact Fluorescent Lamps (CFLs) have much lower wattages while still producing a great amount of light.

Light output is a measure of luminous flux — simply put, the brightness of the lamp — and it is measured in lumens.

More lumens = more light. As a quick reference, an old 60-watt incandescent bulb produces about 800 lumens; a 100-watt incandescent, about 1,600 lumens.

Watts to lumens conversion chart: 40W=450lm, 60W=800lm, 75W=1100lm, 100W=1600lm, 150W=2600lm

Correlated Color Temperature (CCT) — Warm or Cool?

Color appearance, or CCT, is measured in degrees Kelvin (K). Most lighting ranges between 2700K and 6500K.

  • The lower the Kelvin temperature, the warmer the light appears — golden and cozy, like a traditional incandescent bulb.
  • The higher the Kelvin temperature, the cooler the light appears — crisp and bluish-white.

Points of reference: incandescent light sources typically range from 2700K–3500K. Daylight at noon has a Kelvin temperature of about 5000K. CFLs and LEDs are available across the whole range, from 2700K (warm white) up to 6500K (daylight).

Choosing the right color temperature: Warm White / Soft White 2700K–3000K, the standard color of incandescent bulbs; Cool White / Neutral / Bright White 3500K–4100K, good for kitchens and work spaces; and Natural or Daylight 5000K–6500K, good for reading

Color Rendering Index (CRI) — How True Do Colors Look?

CRI (Ra) measures how well a given light source renders color. Scientists evaluate this by comparing 8 reference colors under two light sources: incandescent light (for warm color lamps) and daylight (for cool color lamps).

CRI is represented on a scale from 0 to 100, where 0 is "poor" and 100 is "excellent." The lower the number, the more distorted colors will look under that light source. For homes, look for a CRI of 80 or above; for retail displays, artwork, or anywhere color accuracy matters, 90+ is ideal.

Putting It Together

When choosing a bulb, decide in this order: how much light you need (lumens), the mood you want (Kelvin — warm for living spaces, cooler for task lighting and workshops), and how accurate colors need to look (CRI). Wattage nowadays only tells you what the bulb costs to run.

Questions about a specific application? Contact us — we've been matching people with the right lamp since 1992.