ECEN 4616/5616
Student "#1"
Schlieren photography
1. Introduction
Schlieren photography is widely used in detecting small refractive index perturbation,
especially for transparent objects, such as air, liquid and glass. The basic idea of Schlieren
is to convert phase change, which is much more difficult to observe, to intensity change.
Since detectors, say eye or CCD, are usually sensitive to intensity change, we can use them
with Schlieren for phase observation.
Below are two examples of Schlieren
Left one is wave shock generated by flight. Right picture is a burning candle.
2. Basic concepts
The theory basis of Schlieren is beam propagation in inhomogeneous media.
Refractive index perturbation causes light diverge away, which is proportional to
index perturbation. This is one basis of Schlieren.
ECEN 4616/5616
Student "#1"
Experiment Scheme
In experiment scheme, there are two optical conjugate planes. One is the extended light
source and knife edge. Another one is the test area which is the object, say burning
candle, and the screen.
I focus on two things. One is the knife edge, since it’s an important part of Schlieren. Much
effort is made to make sure that the knife edge is at the exact place to cut off DC light
component. Another thing is to optimize the imaging property of the test area (object).
This is my final design. Two doublet forms an imaging system. Knife edge is between
them. Object is illuminated by plane wave from the left side.
3. Start design
A good start is good for optimization. To minimize chromatic focal shift, I design a
doublet lens, using glass LAK21 & SFL6.
Consider that a Schlieren system usually can be half meter in dimension; I decide to
design a doublet with focal length 100mm.
Calculations:
K1  K 2  K
K  V1 K and K  V2 K
1
2
V1 V2
V1 V2
ECEN 4616/5616
Giving:
Vd (LAK 21)  60.1019
Vd (SFL6)  25.3939
 K1 1.731644K 17.31644
K 2  -0.731644K  -7.31644
For simplicity, I assume the doublet is equiconvex (
K  (n 1)(C C )
1
2
C2 C1
). Using thin lens formula,
,
I get:
R1  73.975367mm R2  -R1
R3  R2
R4  -225.666269mm
I choose visible light as source, since Schlieren is usually performed with sunlight, or bulbs.
The object of Schlieren often has a certain size, say 5 cm, I take 2 fields into consideration.
ECEN 4616/5616
4. Optimization
Variables
Before optimization, I need to choose some parameters for ‘solve’. This is done in LDE.
I make thickness and radius of 1st achromatic doublet to ‘solve’. Also, Zemax can substitute
glass types. In the doublet, there is a ‘pick-up’ solve. This is because I insert a small air gap
between the two lenses, and we need a ‘pick-up’ to make 2 lens surfaces consistent.
The second doublet is the same with the first one, except the glass
types. Reasons:
I just want to minimize the number of variables. I tried to let all parameters change as
variables. It didn’t work well. So, I think 6 or 7 variables are enough for my system.
Merit function
My Merit function is based on Zemax’s default Merit Function.
Rest of PDF didn't convert.
Student "1" wrote a user-defined aperture to simulate the image of a candle, then
combined it with a "cubic phase" surface to put a phase distortion around the "flame"
(simulating index changes due to heat).
Images copied from "Results":
Image of Candle-shaped object, uniform phase:
Image with knife edge -- removes flame part of image:
Image with knife edge and phase distortion centered on flame:
--Parts of flame become visible--