Buried inside a sanitized 1968 procurement memo from the CIA's National Photographic Interpretation Center (NPIC/TSSG/DED) — a document about whether to fund an outside contractor for image analysis work — is one of the most beautiful explanations of a counterintuitive optical principle ever written by a government bureaucrat.

The foreword, intended to help non-specialist readers follow the technical memo, defines the concept of coherence:

Coherence - As related to the Image Analysis Program. Natural illumination is a rabble army of light waves jumbling along out-of-step, bumping into one another, stumbling over some obstacles, and bouncing back from others. However, the very randomness of this incoherent illumination makes it useful for viewing photographic imagery. This is because the variations among the waves tend to average out, giving a statistically uniform light like a crowd moving through Times Square on New Year's Eve.

The forgotten claim hiding inside that gorgeous prose is genuinely counterintuitive: chaos produces uniformity. A coherent, orderly light source — like a laser, in which all photons march in lockstep — is worse for viewing ordinary photographs than the messy jumble of sunlight or a tungsten bulb. The reason is exactly what the anonymous CIA writer intuited: when individual waves are random enough, their fluctuations cancel each other out in the aggregate. Statistical noise becomes statistical smoothness.

This is not folk wisdom. It's the principle behind why coherent illumination produces speckle — those grainy, sparkly artifacts that anyone who has shone a laser pointer at a wall has seen. Every photographic interpreter working with a laser-illuminated lightbox would have to fight that speckle. Incoherent light, by averaging over millions of out-of-phase wavefronts, never does.

What's striking is how the author reaches for two crowd metaphors — a "rabble army" and a Times Square New Year's Eve mob — to explain a phenomenon that physics textbooks usually mangle with phasor diagrams and ensemble averages. The intuition is right: a single drunk staggering down the sidewalk is unpredictable, but ten thousand drunks heading vaguely uptown produce a smooth, predictable flow.

• Modern optical coherence tomography exploits this in reverse, using carefully managed partial coherence to image inside the human eye.
• Engineers building laser projectors today still fight speckle by deliberately destroying coherence — vibrating diffusers, spinning phase plates — to recover the smoothness that the sun gives us for free.
• The same principle underlies why your phone's flashlight makes a better reading light than a laser pointer of identical power.

The deeper lesson, smuggled into a Cold War procurement document, is that randomness is often a feature, not a bug. Statisticians call it the law of large numbers; the CIA writer called it Times Square. Both are right.