Is 35 or 20 tint darker? How 5, 20, 35 and 50% tint look on a car

The percentage is the share of visible light the window lets through, so the scale runs backwards: 20% passes a fifth of the light, 35% about a third, and the lower the number, the darker the window. The definition in full: what the tint percentage means.

The number on the box is the film's name, not the reading on your car. "All automotive glass is tinted to some extent" (IWFA), and the two stack by multiplying: a 50% film over factory glass that passes 82% gives about 41%, because 0.50 × 0.82 = 0.41 (Proffitt et al., 1995). The maker's own figures show it. 3M's Ceramic IR 35 reads 45% on clear glass and 37% on the 73% automotive glass 3M uses as a reference (3M).

Real cars drift further. In two University of Virginia tint studies, the test sedan tinted to read 35% at the front and 20% at the back metered 40% on the front side windows and 13% on the rear side windows (Proffitt et al., 1995; VTRC 96-R14). So a shade is a target, and a tint meter, used on the bare glass before and the finished window after, tells you where you landed.

Less than through clear glass, and how much less depends on where the light is, not only on the number. In a study for the Virginia Transportation Research Council, 320 people looked into identical sedans the way an officer approaches a stopped car. Tint metered at 38% and at 13% on the rear windows significantly cut how well they could count the occupants and spot objects on the back seat. Twilight and night made it harder still, until headlights and a handheld spotlight were shone into the car: then observers found more back-seat objects than they had at midday (Proffitt et al., 1995).

The cabin matters too. The IWFA notes that privacy depends on reflectivity as well as VLT, and that interior color affects how easily occupants are seen, "especially at night": the darker the interior, the darker the film looks (IWFA).

Pros and cons of 20% tint:

• For: strong daytime privacy at the back; 3M's 15 film cuts 78% of glare on automotive glass; it hides headrests and seat tops, which the IWFA counts as part of the look.
• Against: a darker view out at twilight and at night, more so for older eyes (below); much less privacy once light shines into the car; and in California, no use at the front.

In daylight, yes: the same researchers found "no evidence" that reduced transmittance matters in well-lit daytime conditions, when the important things on the road are easy to see. After dark, every step down in VLT makes faint, low-contrast things (a pedestrian in dark clothes, debris) harder to pick out (VTRC 96-R14).

The evidence:

• Mirrors just after sunset. 24 drivers looked for a pedestrian in dark clothes behind the car. Misses at 38 meters or closer rose from 1 in the untinted car to 5, 9 and 20 as the tint got darker (the darkest car's rear side windows metered 13%). The differences were too variable to be statistically significant on their own: tint hurt some drivers far more than others (VTRC 96-R14).
• Age. In a Southern California College of Optometry study, a 37% window did not significantly reduce contrast sensitivity for drivers aged 20 to 29, but did for drivers aged 60 to 69; the younger group lost contrast only at 18% (LaMotte et al., 2000).
• Angle. A tint reading is taken straight through the glass. Over 60 degrees, transmittance drops substantially, and the steepest view is a shoulder check through a rear side window (VTRC 96-R14).

Is 50% tint hard to see at night? Less than the darker shades, but not free: in a driving-simulator study reviewed by the same report, drivers who wore glasses already detected fewer low-contrast targets through a 58% windshield in simulated twilight. Can you still see out of 5% tint? By day, yes. At night it passes a quarter of the light of 20% film, so every loss above grows: headlights still show, unlit shapes far less.

California softens this. The front side windows stay clear, so both side mirrors are viewed through clear glass; only the inside mirror looks through the back glass, and only shoulder checks through the rear side windows. Why ceramic film does not change any of this: the downsides of ceramic tint.

In California this is a choice for the rear side windows and back glass, since the front pair stays clear. Darkness buys privacy and glare control; it says little about heat. On 3M's sheet, its Ceramic IR 35 rejects 58% of total solar energy on 3M's 73% reference automotive glass and its IR 50 rejects 54% (3M), so a few points of heat, while glare reduction drops from 49% to 32%. Film type moves heat more than shade does: what is ceramic window tint.

What is the most popular window tint shade? No maker or agency publishes installation shares. In Los Angeles, the shades people search for most are 70, 20 and 35 (car window tint).