Light observation log

What a white sheet, a gray card and the wall looked like at each hour, and every bulb in the room with its color temperature and CRI.

The log

Each row is one look at one room. The "white paper" and "gray card" columns are the controls: they let you describe the light rather than the paint. "Room in use at this hour?" marks the entries that count; a color must work in those rows first. The "reflected light" column is for whatever is outside the window that tints or brightens the light: a lawn, a brick wall, a neighbor's pale wall, an awning.

Bulbs and fixtures

One row per fixture or lamp. Read the kelvin (K), CRI and lumens from the printing on the bulb's base or from its packaging; write "unknown" where a value is not printed and cannot be found by model number, rather than guessing. Two different kelvin values in one room is a common cause of a room looking wrong in the evening, and the "plan" column is where the fix is written before any paint is sampled. Treat the CRI as a rough guide rather than a verdict: the DOE fact sheet LED Color Characteristics (2012) notes that bulbs with the same CCT and CRI may not render colors the same way, so the rows in the log, not the numbers, are the evidence.

How to use this sheet

• Log at the hours the room is used. If you are home in the evening, the evening rows are the ones that decide. A noon row for a room you are never in at noon is context, not evidence; tick "in use" only on the rows that are.
• Start with the minimum. One look in daylight and one look at each hour the room is mostly used, with a line or two of notes each, is enough for most decisions.
• Go further when it is worth it. For a hard, expensive or contested choice, or when your first rows disagree, add days, try to catch both an overcast and a sunny day (they are two different lights), and look at more than one wall. This longer log is optional.
• Note the sky on every row. If you kept the longer log and only got one kind of weather, say so in the notes and leave the gap honest.
• Keep the same paper and the same card. Printer paper, a napkin and the back of a chip are three different whites. Use one sheet of white paper and one mid-gray card for the whole log, and hold them against the same wall each time.
• Describe, do not grade. "Paper looks faintly blue," "gray card looks brownish by the lamp," "the wall by the window is a different color from the wall by the door." Comparisons are the record; "nice" and "bad" are not.
• Do the bulb sheet first. If it turns up two kelvin values, decide now whether to standardize, and if so do it before the log starts, so that every row is made under the light the room will keep.
• Directions are hypotheses. Write "roughly west (phone compass)" in the facing column, and let the rows say whether the room behaves as the convention predicts.
• Carry the use-hour rows forward. Module 9's sample tracker wants each sample observed at the same hours as the rows ticked "in use" here, so that a sample's behavior can be compared with the room's baseline.