In short
Projection, sillage and diffusion all describe how far a fragrance reaches, and people use them loosely. Define what you mean, then test it with a fixed dose, a fixed surface, an aired room and assessors who do not know which sample is which. A home test detects only large differences, and it should be reported that way.
What you will learn
- Define projection, sillage and diffusion clearly enough to test
- Design a repeatable distance test without skin
- Report results with their limits
Before this lesson: Correlating Sensory and Analytical Data
What you are actually measuring
| Term | Our definition | Rough way to observe it |
|---|---|---|
| Diffusion | How readily the smell spreads through still air from a source | Distance at first detection from a stationary source |
| Projection | How far it can be noticed around a source over time | The same test, at several times |
| Sillage | The trail left by a moving wearer | Hard to test fairly; needs a real wearer and a checked formula |
The Journal's article on these terms explains them for wearers, and the Lab's longevity and sillage guide covers wear testing. Our focus here is making your own test rigorous enough to mean something.
A repeatable distance test
- Choose a surface. A paper strip or a square of clean cotton, the same each time. Do not use skin for an unchecked formula.
- Fix the dose. Weigh it, for example 0.10 g of the working dilution.
- Fix the place. A room with a closed door, no fan, no other smells. Note its temperature.
- Blind it. A helper places a coded source at one end of the room. You enter from the other end, and walk slowly along a marked line.
- Record the distance at first detection, and your confidence.
- Air the room thoroughly, and wait. Materials linger, and residual smell is the largest problem with this test.
- Repeat at least three times per sample, on different days, in shuffled order.
A worked analysis
These numbers are invented, to show how to read a result.
| Trial | Version A | Version B |
|---|---|---|
| 1 | 2.5 | 3.0 |
| 2 | 1.5 | 3.5 |
| 3 | 2.0 | 2.5 |
| Mean | 2.0 | 3.0 |
| Range | 1.5 to 2.5 | 2.5 to 3.5 |
B's mean is higher, but with three trials and one person, this is only a lead. The ranges nearly meet. A fair statement would be: "In three blind trials, one assessor detected B from further away on average. This should be repeated with others." You can look further into sample size in the research track.
What influences the result
- Dose and strength, which is why you weigh them.
- Air movement. A slight draft can outweigh any formula difference.
- Your adaptation and expectation, which is why you blind and take breaks.
- Time since application, so test at a stated time.
- The assessor. Specific anosmia can make a large difference. Your test may mainly show what you can smell.
What increases reach in a formula?
Trade experience points to high-impact and diffusive materials used at suitable doses, such as Hedione and Ambroxan-type materials. But more reach can bring a formula that is loud early and empty later, and it may cause restriction or cost problems. Treat claims of "enhanced projection" with caution unless you can see how they were tested.
Practical exercise: A blind distance test on two formulas
Compare two versions of one composition, one with a diffusive material added by ladder and one without.
- Write your definition, and your prediction, first.
- Run the distance test: three trials each, coded, in shuffled order, on different days if possible.
- Record the means, the ranges and the conditions.
- Write a conclusion in the cautious form of the example above.
- List two things that could have produced the same result without any real difference between the formulas.
You have done this well if the test was blind, your conclusion states its size and limits, and your alternatives are real ones.
Common mistakes
- Testing on your own skin as a shortcut. You will smell it differently, and the formula may not be suitable.
- Testing twice in a row. Residual smell and adaptation will decide the result.
- Treating strength as reach. They are different, as the Level 6 lesson showed.
Key terms
- Projection
- How far a fragrance can be noticed from the source, for a wearer or a test surface.
- Sillage
- The trail of scent left in the air by a wearer. Usage varies, so define it when you use it.
Knowledge check
Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.