In May 1968, a room full of CIA photo interpreters ("P.I.'s") sat down for a briefing on a new imaging technique flying aboard the KH-4 CORONA reconnaissance satellite: bi-color imagery. The briefer immediately corrected the name — it was really bi-spectral imaging — and delivered a warning so nuanced that it foreshadowed one of the most misunderstood concepts in modern remote sensing.

He said that the material taken by this process, if properly understood and/or filtered in the viewing or reproduction process, could be relied upon to indicate whether or not objects were most reflective in the red or green regions of the spectrum, but he cautioned emphatically against interpreting the color presentation of this material as being directly analogous to the actual color of the object.

The document is a memorandum for the record from NPIC (the National Photographic Interpretation Center) — the CIA's photo-analysis shop that pored over U-2 and satellite film. The briefing was a preview: before analysts saw the imagery from a real mission, they were taught what the pictures did and did not represent.

What makes this remarkable is how early it is. In 1968, civilian scientists were still years away from Landsat-1 (launched 1972), the mission that popularized multispectral imaging for agriculture, forestry, and geology. The briefer was already explaining the core epistemological trap of the technique: the colors on the print are a proxy for spectral response, not a photograph of what the ground looked like to a human eye. A field that glows crimson in a false-color composite is not red — it's just reflective in a wavelength band the analyst chose to display as red.

Even more striking is the follow-up exchange, in which an analyst asks whether the KH-4's stereo geometry might create phantom color signatures:

[He] asked whether the convergence of the KH-4 cameras might cause specular reflectance to be misidentified as a "color" signature. [The briefer] replied that this was certainly a potential source of difficulty if the sun angle had the corresponding orientation with respect to the camera attitude.

That question — can a mirror-like glint fool us into thinking we're seeing a spectral property of the material? — is a problem that remote sensing researchers still write papers about today. Modern satellites use bidirectional reflectance distribution functions (BRDFs) and sun-angle corrections to handle exactly this artifact. The 1968 analyst intuited the problem in a single sentence.

• The briefer used Polaroid Land color copies and rectified transparencies as teaching aids — 1968 tradecraft, using consumer instant photography to demonstrate a spy-satellite principle.
• The ARES viewer he mentioned was a specialized instrument for superimposing spectral bands — the mechanical ancestor of today's GIS layer blending.

The forgotten lesson isn't the technique — multispectral imaging is now everywhere, from Google Earth to crop-yield forecasts. The forgotten lesson is the discipline the briefer demanded: never confuse a visualization choice with a physical fact. Every time someone today looks at a NASA "true-color" composite of Mars and assumes it's what your eye would see, they're making the exact mistake this 58-year-old memo tried to prevent.