V-LABTT
Two flops, one of them toward the light
Every colour approval sheet scores the first flop: how an effect paint shines near the gloss angle and darkens as you move away. Crystal Glass Pigments add a second flop. They send light back the way it came, so the paint brightens again on the side of the light source, and the sheet has no number for it. Twin-Flop gives it one, from the angles your instrument already measures. No colour instrument on the market looks straight back at the light yet, so the peak there is calculated from the two angles beside it: an approximation, and labelled as one. Pick a colour and add Crystal Glass Pigments.
Twin-Flop Index
6.6
At 10 % Crystal Glass Pigments, Bursting Blue turns back toward the light. Past 75°, where the plain colour keeps darkening, this one brightens again: 4.6 times the plain colour at 90°, which is the calculated point. Twin-Flop Index 6.6, where the plain colour scores 0.
Daylight shows the first flop. Twin-Flop calculates the second.
Same colour, four loadings of Crystal Glass Pigments, starting with none. The lobe pointing back at the light grows, and the gloss lobe gives some of itself up for it: the light is redirected, not added. The two thin rays are the 75° and 110° on the chart above.
The left of the chart is the first flop, the shine you already buy a colour for. It is an order of magnitude above everything else, which is most of the reason nobody was measuring the second one, and it is why the scale starts logarithmic: on a straight scale the right of the chart is a flat line along the bottom. The right is the second flop, where the orange line climbs away from the grey one, and the index grows with that climb. Turn the log scale off to see it at its true size.
A colour instrument has no detector in its own light path, so it cannot read the second flop’s peak itself. It reads the two angles either side of it, 15° and 20° off, and ΔY90 is how far those two sit above the same colour without Crystal Glass Pigments, weighted toward the closer one: ΔY90 = (4 × ΔY75 + 3 × ΔY110) / 7. Measured, not modelled. The dashed rise is the only part the chart infers.
It is a visibility number, not a pigment count: the light at 90° as a percentage of glossy white, so a light silver scores higher than a dark blue at the same loading. It is not a retroreflection coefficient either: road-sign retroreflection is measured within about 2° of the light, the index uses the angles 15° and 20° away. One part is provisional: the 1.7, set on the one panel series measured so far with a 90° channel.
Y = Σ Float(λ) × W(λ) / 100The BYK-mac i project file holds 31 Float values per angle: the reflectance factor in percent at 400, 410 … 700 nm, relative to a perfect white diffuser. Multiply each by its D65 / 10° brightness weight W (the 31 weights sum to 100), then add the products: one Y per angle.ΔY75 = Y75 − Y75(ref) ΔY110 = Y110 − Y110(ref)How far each shoulder sits above the same colour without Crystal Glass Pigments (the 0 % reference panel).ΔY90 = (4 × ΔY75 + 3 × ΔY110) / 7The excess at 90°, read on the straight line between the two measured angles. 90° is 15° from 75° and 20° from 110°, so the nearer angle, 75°, weighs 4/7 and 110° weighs 3/7.Y90(ref) = Y75(ref) + 15/35 × (Y110(ref) − Y75(ref))The reference’s own straight line between the two angles, read at 90°. A calculated value, never a measurement.Y90 = Y90(ref) + 1.7 × ΔY90The 90° point. Measured when the instrument has a 90° channel. Otherwise ΔY90 is put back on the reference's line, and the factor 1.7 lifts the straight-line value to the top of the retroreflective peak: a provisional calibration constant, fitted on a series with a measured 90° (Hydrolan, BYK R&D lab, 2023), where it agrees within 7 % from 1 to 20 wt% Crystal Glass Pigments.no Crystal Glass Pigments → TFI = 0Whether the coating holds Crystal Glass Pigments is set per sample when matching, not read from the curve. Without them there is no second flop and the index is 0: a plain colour keeps darkening past 75°, whatever the reference.TFI = 100 × (Y90 / Y90(white)) × (Y90 / Y75)The Twin-Flop Index, the headline number: the light at 90°, straight back toward the source, as a percentage of a glossy white panel at 90°, multiplied by how far 90° rises over 75°. With a measured 90° it needs no 0 % panel.With a measured 90°, steps 2 to 5 drop out and no 0 % panel is needed. Without one, the 1.7 in step 5 is provisional, which is why the chart shows a band of 1.5 to 1.8 beside it. Y90(white) is a glossy Traffic White panel (RAL 9016) on the same BYK-mac i model, read on its own straight line at 90°: 85.0. On the Measure tab, Y90 as % of glossy white is also graded A to E (Spectral Visibility SA), with A+ only above 102 %.
Measure a panel set on a V-LabTT, export the project from the colour instrument, and drop both in together. Every sample you match comes back with its own line and its own number, and with a report you can hand to the customer who asked whether it was worth it. Only have the project file? Drop it alone and choose Twin-Flop only. Or try the demo set on the Measure tab first, no password needed.
Example data: Bursting Blue and Lunar Silver, measured on a BYK-mac i, serial 1342731, 2025-02-13. Values are luminance factor Y under D65 at 10°. The index is Y90 as a percentage of a glossy white panel at 90°, times the 90/75 rise, and 0 for a coating without Crystal Glass Pigments. The calculated point toward the light source uses a provisional factor from the only panel series measured so far with a channel pointing that way, and is shown as an indication rather than a reading.