Objective Transmittance

An apochromatic objective contains ten to fifteen lens elements. Every glass-air interface reflects, every element absorbs a little, and the coatings that suppress reflection are tuned for a wavelength band. The result is that transmission is wavelength dependent and often far below the figure people assume: 90% or better through the visible for a good design, but frequently under 50% in the near-infrared.

Set that beside the care usually taken over detectors. Two cameras differing by 5% in quantum efficiency will be compared across a week of demonstrations, while the objective in front of both discards 40% of an 800 nm emission before either sees a photon. System efficiency is the product of the whole chain, not the last term in it, and the transmission curve is the term that is almost never consulted because it is almost never printed on anything.

The shape of a transmission curve follows from what the design was correcting. Apochromats add elements and exotic glasses to bring three or more wavelengths to a common focus, which buys chromatic performance and costs transmission — more surfaces, more absorption, particularly in the UV and the NIR where the glasses themselves become less transparent. A simpler achromat, corrected at two wavelengths, can transmit more light while focusing colors less consistently.

Anti-reflection coatings are the other half. A multilayer coating can drop a surface's reflection from about 4% to under 0.5%, but only across the band it was designed for; outside that band it does much less. Across fifteen surfaces the compounding is dramatic — 4% loss per surface leaves 54% transmitted, while 0.5% per surface leaves 93%. Coatings are most of the difference between a good objective and a disappointing one, and they are entirely invisible on the specification sheet.

Manufacturers publish transmission curves inconsistently and sometimes only on request. It is worth asking, because the alternative is measuring it, and the alternative to that is guessing about a factor that can be two-fold.

You can measure it comparatively without a photometer. Image a stable uniform fluorescent reference — a plastic slide or a dye solution — through each objective at identical excitation, exposure and emission filter, then compare mean intensity, correcting for the NA⁴/M² difference between them. What remains after that correction is transmission. Do it in each emission band you actually use, since a lens that wins in the green can lose badly in the far red.

System detection efficiency is the product of the path: objective transmission times every filter and mirror times the camera's quantum efficiency at that wavelength. Two objectives at 92% and 55% transmission in your emission band, behind identical everything else, differ by a factor of 1.67 in delivered signal.

Compare that with a detector decision. A 95% QE camera against a 90% QE camera differs by 1.06. The objective choice is worth about twelve times the camera choice here, arrives earlier in the chain, and costs nothing extra if you already own both lenses. It is the cheapest large improvement available to most systems, and it is invisible unless somebody goes looking for the curve.