R9 is the score for saturated deep red, and it is not included in the CRI figure printed on the box.
That one sentence explains why two bulbs both marked CRI 80 can make a room look completely different, and why the gap between CRI 80 and CRI 90 feels far larger than ten points.
How CRI is actually calculated
The color rendering index tests a light source against a set of standardised color samples, each with its own R number. The headline figure — Ra — is the average of the first eight.
Those eight are all desaturated pastels: muted pinks, grayish greens, soft blues. They were chosen decades ago as representative of everyday surfaces, and they are easy for a light source to render acceptably.
R9 through R15 are the saturated and specific colors: strong red, strong yellow, strong green, strong blue, and skin tone. None of them counts toward the printed number.
Why deep red is the one that fails
A white LED is normally a blue LED with a phosphor coating that converts part of that blue into longer wavelengths. The result is a spectrum with a hard blue spike and a broad hump across the middle.
The deep red end is the hardest and most expensive part to fill, and it is also the part the eight standard test colors barely probe. So it is the natural place for a manufacturer to economise — the saving is real and the headline number barely moves.
A bulb can therefore reach CRI 80 while emitting almost nothing in the deep reds. An object that reflects only deep red has nothing to reflect, and appears brown or gray regardless of how bright the room is.
What has deep red in it
Far more than people expect:
- Skin. Blood under the surface is why faces go sallow at low R9.
- Cooked meat. Steak, bacon, tomato sauce — all collapse toward brown.
- Wood and brick. Oak, mahogany, terracotta, exposed brick.
- Red wine, fruit, flowers, leather, rugs, most warm-toned textiles.
That list is a large fraction of what a living room and a kitchen contain, which is why R9 has an outsized effect on whether a room feels good.
| R9 | Verdict | Effect |
|---|---|---|
| 0–20 | Poor — common at CRI 80 | Skin sallow, reds go brown |
| 20–50 | Marginal | Noticeable on wood and food |
| 50–80 | Good — typical of CRI 90 | Reds honest, skin natural |
| 80+ | Excellent | Vanity, dressing, art |
What to look for
R9 above 50 for any room you live in. Above 80 for a bathroom vanity, a dressing area or anywhere art hangs.
The practical problem is that most retail listings do not print it. Two rules deal with that:
A manufacturer who publishes R9 usually has a good one. It is a selling point; nobody advertises a weakness.
Silence means assume the worst. A product that quotes CRI 80 and nothing else is very likely in the 0–20 band.
Products aimed at photography, video, retail display and horticulture almost always publish full R-values, because their buyers check. Those are often the same LEDs sold at higher margin into the domestic market without the numbers attached.
The trade-off
Filling in the deep red costs efficiency. A high-R9 bulb typically gives up 5–15% of its lumens per watt compared with a CRI 80 equivalent of the same color temperature.
On a 9 W bulb running four hours a day, that difference is roughly one watt — a cost so small against the visible improvement that it is not worth thinking about in a living space. In a loft or a garage, take the efficiency instead: the recommendation by room is in what CRI you need at home, and what the two common grades look like side by side is in CRI 90 vs 80.