Doppler and Optical, Side by Side in One Bay
We run both on every swing, which puts us in an unusual position to say what each one is genuinely good at. The short version: they are not competitors, and a shop that owns only one is guessing about half of what it reports.
Arguments about launch monitors are usually framed as brand loyalty. That framing obscures the actual engineering question, which is not "which device is better" but "which physical quantity is this device observing, and which is it calculating from something else". Once you sort the outputs into those two piles, most of the argument dissolves.
How each one sees the shot
A doppler system emits a radio signal and reads the frequency shift of the returns as objects move through the beam. It is watching the ball, continuously, from behind. That gives it a genuinely excellent view of what the ball is doing over distance — velocity, and how spin decays as flight progresses.
An optical system takes an extremely fast sequence of images through the impact interval and works out positions and orientations from the frames. It is watching a very short window with very high spatial precision. That gives it an excellent view of what the club is doing at the moment it meets the ball.
Different sensing principle, different strength, different failure mode. Neither is a cheaper version of the other.
The direct measurement split
Here is what each system observes directly rather than deriving:
- Doppler measures directly: ball speed, launch angle, spin rate, spin axis, and the trajectory of the ball for as long as it stays in the beam.
- Doppler infers: club head speed, club path, face angle, and attack angle. All of these are reconstructed by working backwards from what happened to the ball.
- Optical measures directly: club head speed, club path, face angle, attack angle, dynamic loft, strike location on the face, ball speed and launch angle.
- Optical infers: spin axis, and anything about flight beyond the first few inches.
Spin rate is the interesting middle case. An optical system can measure spin very accurately, but it needs something on the ball to track — markings, a logo, or a dimple pattern it can resolve. Give it a scuffed range ball with no clear features and its spin figure degrades quietly rather than reporting an error. That is a failure mode worth knowing about.
Where indoors changes the picture
Outdoors, doppler has room to work. The ball flies, the system tracks it for hundreds of yards, and its spin and flight data are collected across the whole trajectory. This is the environment it was designed for and it is very hard to beat.
Indoors, that advantage largely disappears. The ball travels a few metres before hitting a screen, so the system gets a truncated look and has to extrapolate from a short observation window. Modern units handle this well, but "handles it well" is a different claim from "measures it".
Optical does not care. Its entire measurement happens in the first fraction of a second after impact, and a screen three metres away is irrelevant to it. Indoors, the optical system's advantage on club delivery becomes larger, not smaller.
This is the core of the argument for pairing them. Indoors you want the optical system leading on club delivery and the doppler system leading on ball behaviour, with each providing a check on the other's inferred values.
What disagreement tells you
We hold tolerances: 400 rpm on spin, 0.8 degrees on launch angle. When a strike exceeds either, the strike is discarded and the disagreement is noted. Over several years we have found the causes cluster into four groups.
Off-centre strike. By far the most common. The optical system reports where on the face contact occurred; when that is toward the heel or toe, gear effect changes the spin axis in a way that stresses the inference chain on both units. The disagreement is a symptom, not a fault.
Ball feature degradation. The optical system's spin reading needs trackable features. A ball that has taken several sessions of abuse produces a spin number that drifts from the doppler reading. We replace reference balls on a fixed schedule specifically to remove this variable.
Genuine calibration drift. Rarer than you would expect, but it happens, and it is exactly what a paired setup is for. A one-sided systematic offset appearing across many strikes and many golfers means a unit needs attention. Grant runs the check every operating morning.
Extreme delivery. Very steep attack angles, very open or closed faces, and unusually low ball speeds all push both systems toward the edges of their reliable range. Junior sessions and very slow swing speeds produce more of these than anything else.
What this means for a fitting
If a shop runs only a doppler unit indoors, its face angle and club path figures are inferences from a truncated ball observation. They are often good inferences. They are not measurements, and there is nothing available to check them against.
If a shop runs only an optical unit, its spin figure depends on ball condition in a way that fails silently, and it has no independent view of what the ball did after the first few frames.
Running both means every important number has either a direct measurement or a cross-check behind it. It costs more, it takes longer, and it occasionally produces the answer "we cannot make a confident recommendation from today's data". We regard that last outcome as the whole point.
What we do not claim
Pairing does not make indoor carry distance a measured quantity. It is still modelled, still dependent on air density and ball, and still an estimate. What pairing improves is the quality and verifiability of the inputs to that model, which is a real improvement but a narrower one than "our numbers are more accurate" would suggest.
The measurement guide lays out which figures on your sheet are measured, which are inferred and which are modelled, by system. It is worth reading before your session rather than after.