Copyright – P. J.
Smith
Ronchi’s
First Steps
Vasco Ronchi was born 19/12/1897 in Florence.
His study of Physics was interrupted by WW1.
1922 – He published a method of testing large optics
quickly with small gratings. Much later
this generic type of test was called the Ronchi test. We often forget now that fewer methods then existed to do this.
Most of the interferometers that could do more complex testing were yet to appear
commercially on the scene.
Optics was originally not his main area of
expertise. He had wide interests, both
scientific and humanitarian.
He is probably most known for his history and study of
light and vision, especially with respect to wave theory. Less well known were his endeavours in
organising Optical Institutes - both for research and production. These ranged from precision optics to
Ophthalmology.
In 1953 he was elected President of the 'Union
Internationale d'Histoire des Sciences' within UNESCO and was re-elected on
four subsequent 3 year terms.
He was nominated as 'Fellow Emeritus' of the Optical
Society of America.
After publishing about 900 papers and 30 books, he
died 31/10/1988.
Most of his papers were published in Italian so much
historical information about the Ronchi Test we obtain second hand from papers
published in English describing and summarising his work.
Most of his papers were published in Italian so much historical
information about the Ronchi Test we obtain second hand from papers published
in English describing and summarising his work.
Vasco Ronchi is usually credited with the introduction
of the Ronchi test in the early 1920's. The theory and practice of this family
of tests is almost totally due to his persistence. Later it was Schulz who coined the name 'Ronchi Test'.
Vasco usually referred to what we now call the 'Ronchi
Test' as a grating interferometer and had an obsession with showing that it was
a true interferometer. A lot of his
work was slanted to supporting the wave nature of light and reconciling this
with the Ronchi test so many of the papers on this topic are hard reading.
In an article "Forty Years of History of a
Grating Interferometer" [1]
which Vasco Ronchi wrote for Applied Optics in 1964, he gives a comprehensive
resume of articles which have appeared in print and by the time the Anderson
and Porter paper was published in 1929,
26 papers had already been published elsewhere on the topic of which 21
are by Ronchi himself. Most of the very early history is taken from this source
and will not be re-credited.
The first Ronchi
test.
“R. A. Occhialini, who was working at the Institute of
Physics of the University of Florence (where the author was continuing his
studies after having obtained his University Degree) had thought of utilizing
gratings as a means of carrying out precision measurements with very high
sensitivity precision. Occhialini’s
attempts did not give very interesting results and he decided not to dedicate
any more of his time to gratings.”
“Having assisted him in these experiments, the author
found himself in possession of some gratings obtained photographically, with 3
or 4 lines/mm, that Occhialini left at his disposal.”
“Towards the end of 1921 a concave spherical mirror of
almost 15 cm radius of curvature was sent to the institute where the author was
working, for a determination of this radius with the greatest possible
precision.”
“In order to accomplish the required measurement, we
decided to determine the position of the centre of curvature of the concave
surface by means of the gratings left to us.
During his research, Occhialini had noticed that by overlapping two gratings
with different frequencies or even two equal gratings, but one inclined in
respect to the other, some well known fringes were formed, commonly called
”moirés” but which he called combination fringes.”
“We decided to place a material grating at the same
plane as the real image given by the spherical mirror; the combination fringes
were expected to disappear entirely when the plane of the grating had passed
the centre of curvature, because the grating image should have coincided
exactly with that object.”
“Because the mirror had a very small radius of
curvature, it was very hard to align properly so he carried out experiments
with a larger (D = 50 cm, Radius of curvature = 100 cm.) low precision mirror.”
“Having placed the material grating where more or less
the centre of curvature of the mirror was to be found, we examined it from a
distance of about 20 cm. To observe the combination fringes that were to give
an indication whether the grating was near or far from the centre itself; but
it was not satisfactory because the fringes were almost undecipherable.”
“Then by chance it happened that, perhaps with the
intention to see the desired fringes better, the author put his eye close to
the grating so that he could see through it the whole mirror. Thus he saw the surface of the mirror lined
with a number of twisted and irregular fringes that immediately made the
serious faults of workmanship noticeable in nearly concentric zones of the
mirror. We immediately felt that the
grating thus employed could be used to point out defects in workmanship of a
concave spherical mirror.”
“We immediately undertook an analysis of the
phenomenon, so that it could be used as a rational, systematic, and also, if
possible, quantitative application.
From this the grating interferometer for testing optical systems had its
start.”
“The method was promptly adopted for lenses and was
much simplified; the reasoning that immediately presented itself with evidence
brought forth the employment of a point source.”
Apparently, the work to measure the original mirror's parameters
was never completed. But the idea fortuitously was applicable to the quick
testing of some large refractor objectives (originally by the famous Amici
which Ronchi had laboriously tested using the Hartman test, also some by
Galileo), so many of his papers were specific to this application and
influenced by his previous experiences with laborious testing of large
objectives.
This was way
before any other type of shearing interferometer (50’s and 60’s) and the shapes
of the fringe pattern in the presence of spherical aberration was something
new. While Ronchi was a little
disparaging of over zealous geometrical interpretations of these fringe shapes,
we know now that a rigorous geometrical analysis would have explained these
fringe shapes.
“A period of evolution of these ideas followed, and
the fact that, in the meantime in other centres of optics, gratings were being
employed testing optical systems also contributed to this process. The most interesting cases were those of
Lenouvel and of Schulz. Schulz’s
publication, however, did not appear until 1928. We mention these cases because it must be stated that they really
did not speak of gratings, in the meaning of instruments functioning by
diffraction; Lenouvel, instead of speaking of ‘resaux’, speaks of ‘trames’, and
Schulz refers to ‘Rastern’ instead of ‘Gitter’. This philosophical remark is sufficient to demonstrate that these
authors were only interested in gratings ‘at low frequency’, therefore, there
was no need to refer to interferential considerations, but the strictly
geometrical study of their behaviour was sufficient. These authors explicitly limited themselves to gratings of not
more than 10 lines/mm.”
“Particularly interesting was the publication of Yvon
who, by taking the geometrical reasoning too seriously, reached unfavourable
conclusions on the employment of gratings as a means of study of optical
systems.”
“As long as the grating employed had a very low
frequency, like the ones we had used at first and that also had been used by
other authors treating the same argument, the geometrical reasoning
corresponded quite well with the results of the experiments and measurements;
but at the same time the method did not lead to results as fine as desired. It was evident that in order to increase
this sensitivity it would be necessary to use gratings of the highest frequency
possible, but then the results decidedly deviated from those predicted from
geometrical reasoning.”
“ --- gratings of still higher frequency were
required, reaching the maxima allowed by theory; whereas at first there was a
tendency to use gratings of 10 lines/mm, these were substituted by 50 lines/mm,
and then 100, 150, and even more.”
It is interesting that Malacara’s research disputes
some of the need for finer and finer gratings.
He actually states that the sensitivity with respect to analysis of
spheres is independent of grating frequency but not so for aspheres.[2] This supports my own experiences that, if a
coarse grating is drawn close to focus to increase sensitivity, and scanned to
cover the whole surface, sensitivity to visual analysis is as good as using a
finer grating.
Ronchi briefly mentions “the development of the phase
contrast grating”. It is unclear if he
is referring to this in general, or specifically with respect to the Ronchi
Test.”
Publications
during this era
1920 R. V. Occhialini. Riv. Ottica. meccan. precis. 1, 99 (1920)
1920-21 W. Shackelton. Trans. Opt. Soc. 22, 167,
(1920-21) "The Testing of Heliograph
Mirrors and the Measurement of Mirrors of Long Focal Length"
1922 V. Ronchi, Riv. Ottica. meccan. precis. 2, 19,
(1922)
1922 V. Ronchi, Riv. Ottica. meccan. precis. 2, 9,
(1923)
1923 V. Ronchi. Ann. Scuola Normale Superiore di Pisa.
15, (1923) Ronchi's original paper on
Ronchi Test. Unknown exact Page or title ?
1923 Y. Vaisala. Ann. Fenn. Aboensis Sarja Ser. A 1,
No. 2, (1923) "Neue Methoden zur
Untersuchung der Objektive" (New
Method of Testing Objectives)
1923 V. Ronchi. Rend. Accad. Naz. Lincei 32, 162,
(1923)
1923 V. Ronchi. L'Universo. 4, 10, (1923)
1923 V. Ronchi. Rend. Accad. Naz. Lincei 32, 339,
(1923)
1924 V, Ronchi. Rend. Accad. Naz. Lincei. 33, 23,
(1924)
1924 V. Ronchi. L'Universo, 5, 2, (1924)
1924 V. Ronchi. Rend. Accad. Naz. Lincei. 33, 314,
(1924)
1924 V. Ronchi. Nuovo Cimento 1, 209, (1924)
1924 L. Lenouvel. Rev. Opt. 3, 211, (1924)
GOTO RONCHI INDEX