Emission: Neon, LEDs, and Gas-Discharge Light

There are two ways for something to have color. Reflective color - an apple, a wall, a printed page - starts with a light source and subtracts: the surface absorbs some wavelengths and bounces the rest. Emissive color - a flame, a star, a neon tube, an LED - adds light into the world that wasn't there before. The color of an emitter is simply the shape of the spectrum it produces.

And those shapes vary wildly. The Sun and a hot filament give a smooth continuous spectrum across all visible wavelengths. A gas-discharge tube gives a handful of razor-thin lines. A fluorescent tube mixes lines with phosphor bands. A white LED is a blue spike with a broad phosphor hump. Each shape can look "white," or colored, and - crucially - each renders the objects it illuminates differently.

Energize a thin gas - run a current through it - and its atoms jump to higher energy levels, then drop back, releasing the difference as photons. Because an atom's energy levels are quantized, only specific energy gaps exist, so only specific wavelengths come out: sharp, bright emission lines. The pattern is unique to each element - a barcode you can read to identify it across the galaxy. It is also why each gas tube has its own signature glow. Pick an element and see its lines and color.

How does a set of separate wavelengths become a single perceived color? Your three cone types each respond to the whole bundle at once and report just three numbers - so the brain can't tell a mixture of lines from a smooth spectrum that excites the cones the same way. Two narrow lines, a red and a green, land on the eye as yellow - a perfect metamer of a single yellow wavelength, even though no yellow light is present. Slide two lines and watch them fuse.

There is no such thing as a "white" LED chip. The white light in your home is a clever fake: a blue LED (a narrow spike near 450 nm) is coated with a phosphor that absorbs part of the blue and re-emits a broad band of yellow-green. The blue that leaks through plus the phosphor's yellow add up to white. More phosphor and the white goes warm; less and it goes cool and bluish. The tell-tale sign is the dip between the blue spike and the phosphor hump - the "cyan gap" that limits color rendering. Build one.

Here is the payoff. Because a surface can only reflect what the light contains, the spectrum of your light source quietly rewrites the color of everything it touches. Under smooth daylight, all colors render faithfully. Under a low-pressure sodium lamp - one yellow line - reds, greens, and blues have nothing to reflect and collapse into yellows and grays; that orange motorway glow is monochrome. The Color Rendering Index (CRI) puts a number on this. Switch the light over a row of objects.

Every artificial light you meet is one of a few spectral shapes. Knowing the shape tells you the color it glows and how well it renders.

Six questions on emission, line spectra, white LEDs, and color rendering. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Emission is one half of every color story; here is where it connects to spectra, measurement, and the light that fills a room.