Optical Transfer Function Gets a National Standard: Specs Finally Become Comparable

On 28 September 2026 the State Administration for Market Regulation approved four national standards in optics, all taking effect on 1 April 2027.

The four are:

GB/T 12085.3-2026, Optics and photonics — Environmental test methods — Part 3: Mechanical stress;

GB/T 12085.23-2026, Optics and photonics — Environmental test methods — Part 23: Combined low pressure and low temperature, atmospheric temperature, high temperature or damp heat;

GB/T 4315.1-2026, Optical transfer function — Part 1: Terms and symbols;

GB/T 4315.2-2026, Optical transfer function — Part 2: Measurement guidelines.

The standards fall under Technical Committee TC103, the national committee for optics and photonics standardisation. Part 1 of the optical transfer function standard was drafted jointly by sixteen institutions including the University of Shanghai for Science and Technology, MOTIC, Sunny Optical, Yongxin Optics and Changchun University of Science and Technology, with MOTIC leading the drafting work.

1. What the optical transfer function measures

When buying a microscope, most people compare magnification and resolution. Both are single numbers, and neither describes the imaging quality of the whole instrument.

The optical transfer function describes a curve. Feed the lens patterns of varying fineness, expressed as line pairs per millimetre, and measure how much contrast survives. Spatial frequency runs along the horizontal axis, contrast transfer from 0 to 1 along the vertical. That curve is the MTF, the modulation transfer function, which is the magnitude of the OTF.

Its value is that it covers the whole frequency range. Resolution tells you only the finest detail that can be separated; the MTF curve tells you how much contrast remains at every level from coarse to fine.

Consider two microscopes, both specified at 0.5 micrometres resolution. Instrument A holds a flat curve through the mid frequencies and falls only near the limit. Instrument B has already lost half its contrast in the mid range. Looking at the same slide, A appears clean and B appears hazy. The datasheets look identical; the MTF curves separate them immediately.

This is why the optics industry treats MTF as the authoritative measure of imaging quality, and why it is the shared language of camera lenses, telescopes and lithography objectives.

2. Why terms need a standard of their own

Because until now there was no common definition.

Two manufacturers can both report MTF while measuring under entirely different conditions: white light versus 546 nanometre monochromatic light, centre field versus 0.7 field, contrast transfer function converted versus directly measured. The resulting numbers can differ by a wide margin.

Without agreed terms and symbols, parameters cannot be compared across products, and buyers are left with impressions or price.

Part 1 settles what things are called and how they are defined. Part 2 settles how they are measured and under what conditions. Together they make an MTF report comparable.

3. What the environmental standards cover

The GB/T 12085 series defines environmental test methods for optical instruments. The two new parts address mechanical stress and combined climatic conditions.

Mechanical stress means vibration, shock and drop during transport and handling. A microscope travels from factory to laboratory through logistics, stairwells and unboxing. Whether the optical axis shifts and the focus mechanism loosens is precisely what this test answers.

The combined low pressure, low temperature and damp heat test corresponds to real conditions: high altitude, northern winters, southern rainy seasons, tropical climates. Optics suffer most from damp heat, which drives fungal growth on coatings, separation of cemented elements and corrosion of mechanical parts.

For buyers, the point is that an instrument claiming compliance is one that holds its performance through transport and climate, not only in a temperature-controlled laboratory.

4. What it changes for procurement and acceptance

First, parameters become comparable. Given two product sheets, both reporting MTF curves measured under the GB/T 4315.2 guidelines, a direct comparison is finally possible.

Second, tender documents can cite the standard. Requiring that imaging quality be measured according to GB/T 4315.2-2026 carries far more weight than asking for "clear imaging".

Third, acceptance testing gains a tool. Previously acceptance amounted to asking whether the image looked clear, which is subjective. Now an MTF dataset measured to the national standard can form part of the acceptance criteria.

5. Where MOTIC stands

MOTIC is one of the principal drafting organisations for Part 1 of the optical transfer function standard, and serves as vice chair of the microscopy subcommittee under TC103.

That is worth noting, but worth noting soberly. Participating in drafting indicates that a company's technical team has a voice in the industry and that its product line generally aligns with the standards. It does not mean every product it makes is the best available.

What matters in the end is the measured data for the specific model in front of you.

6. A closing note

The standards take effect next April, not tomorrow. But the direction is clear: optical instruments are moving from "how many times does it magnify" toward "what curve does it actually produce".

That is good news for manufacturers with real capability and bad news for those who invent specifications. It is better news still for buyers, who will no longer have to rely on a persuasive sales pitch alone.