1 Introduction

Two approaches are possible for measuring spectral transmittance: a) using a spectrometer, and b) using a monochromator and a light source.

Approach a) compares the broad spectrum from a light source, before and after going through an object. After passing through the object, wavelengths are “separated” in the measuring device, a spectrometer. Approach b) uses monochromatic light of many different colours (narrow wavelength ranges), produced by means of a monochromator and a light source, and a broad band sensor is as measuring device. The wavelengths are separated in the light source before passing through the object.

For a relative characterisation, it is enough to compare the shapes of the “before” and “after” spectra. For an absolute measurement, the total amount of radiation needs to be compared. Measuring the transmittance of a lens is not as straightforward as measuring the transmittance an optical filter. The lens will modify the path of the light, making it more difficult to collect all the light exiting from it.

A spectrometer is a measuring device, consisting in a monochromator and a sensor array, or a single sensor onto which light of different wavelengths are projected onto to be measured. In contrast, a light source with a broad emission spectrum followed by a monochromator that separates the different wavelengths or colours and projects them onto the object under study. Sources of high irradiance monochromatic light are very expensive devices, even more expensive than spectrometers, which are not cheap at all.

Are there any cheaper alternatives? Yes, in recent years sets of small filters have been used to assess the spectral transmittance of objectives in the UV region (see Savazzi n.d. and references therein). A board is prepared with holes covered with narrow-band-pass interference filters. Filters are illuminated from behind by means of a strong broad-band light source such as a special or modified flash or the sun and photographed through the objective to be tested. Of course one needs a camera sensitive to the wavelengths to be tested, such as a “full-spectrum” converted mirror-less or DSLR camera. This is approach b) modified by replacing the monochromator by a set of filters and the broadband sensor by the camera. For comparable results, the light source should provide a reproducible amount (irradiance) and colour (spectrum) of radiation. The advantage of this method is that a single photograph is enough to qualitatively characterise an objective.

I decided to try a different variation of approach b): to replace both the broad band light source and the monochromator by LEDs emitting at different wavelengths, using a camera as measuring device. My hope was to achieve semi-quantitative and reproducible measurements. I tested 13 objectives, each one in combination with four different filters, and without a filter. I used as a target a slab of white PTFE (“Teflon”), as this thermoplastic has very high reflectance in the range of interest, between 300 nm to 1200 nm (Depending on purity grade, thickness of the slab and cleanness of the surface up to 99% reflectance).

The present tests aim at assessing the UV transmittance of the set of fixed-focal-length (or “prime”) objectives I had available. I acquired a few of them based on the expectation of their usefulness for ultraviolet photography while others I had acquired for other uses. Although good transmittance is a requirement for successful UV photography, other properties, not tested here contribute to usability. The modern Sigma objectives from the DN Art series tested, for example perform much better than other objectives in terms of image resolution when used with a large diaphragm apertures. In practice this means, that even if of lower transmittance, they can be used at higher shutter speeds. On the other hand, objectives that have higher transmittance at shorter wavelengths, will be more suitable for false-colour processing as there will be stronger signal in the red channel, at least in sunlight. Consequently, which of these accidental objectives will be most suitable will depend on the intended use, including which filters they are paired with and what type of final rendition of the photographs is desired. The testing approach used here is only semi-quantitative, but takes into account the camera and filters used.

2 Methods

2.1 Equipment

For the tests I used an Olympus E-M1 camera converted to full spectrum by DSLR Astrotec. This camera is a mirror-less camera with a Micro Four-Thirds lens mount and a 16 mega pixel sensor with a crop factor of 0.5.

I tested 13 objectives, of which seven were released after year 2000 (six of which are currently in production) and the remaining five are designs from the 1960’s and 1970’s and manufactured in the 1970’s (Table 1). All the OM Zuiko objectives are early production types, in “silver nose” livery (later “black nose” versions of the same objectives are thought to be all multi-coated) (described at the “Olympus Shared Resources” web site). The Soligor 35 mm and the Hanimex 35 mm have a coating giving blue reflections. They are of the type described as being based on the same optical design at the Petri Kuribayashi “Kuri”, Kyoei, Acall and similar (Hanimex and Soligor described by Savazzi n.d.). The barrels are different. The four Sigma objectives tested in their MFT (micro four-thirds mount) version are also available in Sony E mount (Sigma 19 mm described by Savazzi n.d.). All Olympus OM objectives were tested mounted on the E-M1 with a Novoflex MFT/OM adapter. The Soligor objective has an M42 mount, and was mounted on the E-M1 with a Fotasy M42 to MFT adapter with focusing helicoid. The Hanimex objective has a Minolta MD mount and was tested mounted on MD to MFT adapter with focusing helicoid (unbranded).

The light transmission properties of the glass or plastic from which optical elements are made of, and the length of the light path through these materials affect the transmittance of an objective. In addition, at each air-glass or air-plastic interface some light is reflected. Lens elements are frequently arranged in groups with elements affixed together, in which case the material used to affix the elements together will also affect the overall transmittance. From this we can surmise that the more complex a lens design is, the less likely it is to have high transmittance outside the range of wavelengths it was designed for.

Reflections are controlled by means of anti-reflection (AR) coatings, of which there are many different variations. Coatings can be optimized for different wavelengths, and the material used for coatings can potentially absorb radiation. Single anti-reflection coatings usually are made of magnesium fluoride, which is transparent to ultraviolet radiation. However, AR coatings can increase reflection outside the target range of wavelengths for which they are designed. In addition, objectives may have different coatings on different lens-element surfaces, and rarely, even in modern designs all glass-air surfaces internal to an objective are coated. So although, testing/screening old objectives of simple design, lacking AR coating, or having only single AR, is more likely to yield good accidental UVA-capable objectives, it is not impossible that some newer simple objective designs are also UVA-capable by accident. In general good prime objectives (single focal length) with moderate maximum aperture can be produced using much simple designs than zooms or objectives with large maximum apertures. In addition, primes of very short focal length, tend to use complex designs. In Table 1 available information on the optical design, coating and year of introduction and/or production are given for all the objectives tested.

Table 1. Details about the objectives tested. Column “objective” gives the abbreviated code used in figures to distinguish the objectives. The year of introduction to the market is when know, accurate, while the year of production is in several cases estimated from the date of purchase, so some of the newer objectives could have been manufactured before the date given, but never later. The values in column “vis.factor” as estimated differences in transmittance assessed by the response of the red sensor channel to a white target illuminated with warm-white LEDs. The Soligor objective is dated based on the serial number using the tables at the Apotelyt site as a guide
read_csv("objectives.csv", comment = "#") %>%
  mutate(objective = factor(objective),
         mount = toupper(mount),
         vis.factor = signif(vis.factor / 1.367, 3L)) ->
kable(arrange(objectives.df, introduction))
objective mount make focal.length aperture.max elements groups coating sn introduction produced vis.factor
z50 OM Zuiko 50 3.5 5 4 single 124064 1972 NA 0.860
z100 OM E.Zuiko 100 2.8 5 5 single 139257 1972 NA 0.878
z35 OM G.Zuiko 35 2.8 7 6 single 153937 1972 NA 0.732
z28 OM G.Zuiko 28 3.5 7 7 single 116979 1972 NA 0.732
dz50 FT D.Zuiko 50 2.0 11 10 multiple 010210916 2003 2008 0.985
mz60 MFT M.Zuiko 60 2.8 13 10 multiple ABQA20394 2012 2015 1.130
s30.28 MFT Sigma.DN.A 30 2.8 7 5 multiple 50107246 2013 NA 0.732
s19.28 MFT Sigma.DN.A 19 2.8 8 6 multiple 51633510 2013 2016 0.683
s60.28 MFT Sigma.DN.A 60 2.8 8 6 multiple 52453077 2013 2017 0.819
mz25 MFT M.Zuiko 25 1.8 9 7 multiple 346022424 2014 2015 0.732
s30.14 MFT Sigma.DN.C 30 1.4 9 7 multiple 51824954 2016 2017 0.732
solig M42 Soligor 35 3.5 5 5 single 9700253 NA 1970 1.000
hani MD Hanimex 35 3.5 5 5 single 13368 NA NA 0.732

I used four different filters: two short pass UV transmitting filters and two long pass UV-absorbing filters (Table 2). To speed-up testing I used Manfrotto’s Xume magnetic filter attachment rings.

Table 2. Description of filters used.
read_csv("filters.csv") %>%
  mutate(filter = factor(filter)) ->
filter make type size supplier acquired
baader Baader U-Venus 48 mm Baader planetarium 2015
edge UVROptics StraightEdgeU 52 mm UVROptics 2016
fire Firecrest UV400 52 mm Formatt Hitech 2017
haze Tiffen Haze 2A 52 mm Tiffen 2017

I used four different types of narrow band LEDs, with nominal emission maxima at 405 nm, 385 nm, 365 nm and 340 nm, plus a warm white LED (Table 3). The actual position of peaks of emission differed slightly from the nominal values. The white LED was used to assess the inherent transmittance of the objectives at the f-stop setting used in the tests.

Table 3. Column “led” gives the nominal emission maxima, while “wavelength” gives the measured emission maxima. “current” indicates the current setting used during testing, which for some LEDs was below the maximum rating given in the corresponding data sheet. “optical power” gives nominal values from the data sheet, but real values for individual LEDs can be expected to differ. “bean angle” are also those given by the manufacturers.
read_csv("leds.csv") %>%
  mutate(led = factor(led)) ->
led wavelength make current nominal power optical power beam angle type
340nm 345 nm Marktech 0.5 A 2 W 55 mW 110 MTSM340UV-F5120
365nm 368 nm LED Engin 0.7 A 3 W 1.20 W 70 LZ1-10UV00-0000
385nm 388 nm LED Engin 0.7 A 3 W 1.15 W 68 LZ1-10UB00-00U4
405nm 405 nm LED Engin 0.7 A 3 W 1.05 W 68 LZ1-10UB00-00U8
warmw 2700 K CRI 90 NICHIA 0.7 A 2.9 W 85 lm 110 NS6L183AT-H1

The partial overlap of the emission spectra of the different LEDs, gives this method much lower wavelength resolution than using a monochromator-based light source. On the other hand, using light sources and a camera allows testing the “camera + objective + filter” performance which is of interest. This is possible as camera sensors have smooth response spectra with broad peaks.

four_leds.mspct <- leds.mspct[c("MTSM340UV_F5120","LZ1_10UV00","LZ1_10UA00_U4","LZ1_10UA00_U8")]
four_leds.mspct <- normalize(four_leds.mspct)
four_leds.mspct <- clip_wl(four_leds.mspct, range = c(300,500))
four_leds.spct <- rbindspct(four_leds.mspct, idfactor = "LED")
fig <-
plot(four_leds.spct, annotations = "peaks") +
  aes(linetype = LED)

Figure 1. Emission spectra from the narrow-band LEDs used, normalized to the wavelength of maximum emission.

Figure 2. Emission spectrum from the broad-band “warm-white” LEDs used, normalized to the wavelength of maximum emission.
plot(normalize(leds.mspct$NS6L183AT_H1), = VIS())

A slab of white “virgin” PTFE (150 mm \(\times\) 150 mm and 3 mm thick) was used as a target. It was cleaned and its surface made matt by sanding it under running water with wet/dry sandpaper (grit P2000). The target slab was illuminated for each test, with two high-power LEDs, with a broad beam (68 to 110 degrees). LEDs were firmly attached to the same tripod as the camera, so as to avoid changes in illumination. The LEDs were driven by constant current with regulation better than ±1% using a DC/DC LED driver with built-in voltage reference (RECOM RCD-24-0.70/Vref adjusted by means of a two decades precision potentiometer, Bourns 3682S-1-103L-ND). This ensured very even illumination of the target, minimal variation with time and easily repeatable settings. As a power source either a 12 V 6800 mAh Li-Po power bank or a 12 V DC 4 A power supply were used (Leicke AC Adapter NT03011).

All LEDs used where obtained ready mounted on star-boards (manufacturer’s part numbers given in Table 3). They were all individually attached onto heatsinks (Ohmite SA-LED-113E) using pre-cut double-sided heat transfer patches (t-Global Technology LP0001/01-L37-3F-0.25-2A). Each heat sink was attached with two black Nylon M5 screws to a custom-cut and bored black Nylon-6 plate. A 25 mm Arca plate was attached with a 1/4 inch (UNC) screw to the back of the nylon plate.

The camera was supported on a carbon-fibre tripod (Giotto’s MTL8361B, similar to current model YTL 8383) with a ball-head (Sirui K-20X) to which a macro focusing rail (Sunwayfoto MFR-150) was attached and then the camera attached to it. On the back of the focusing rail a small Arca clamp (Sunwayfoto DDC-26) was used to hold a “magic ball” (iShoot IS-MSQ) to which two 25.4 cm “magic arms” (Aputure A10) were screwed (Tarion 27 cm magic arms were used until they could be replaced by better ones). At the end of each arm a “fish-bone” Arca clamp (generic, similar to Mengs photo FC-SK25) was tightly screwed and used to hold the LED assemblies through a 25 mm Arca plate (generic, similar to Mengs PU25).

The camera shutter was triggered remotely with a wireless trigger (Hähnel Captur Receiver O/P triggered by a Captur Module-Pro remote, used manually). Using a wireless trigger not only avoided camera shake, but allowed me to trigger the camera from a distance of 1.5 to 2 m so as to avoid disturbing the illumination.

Photograph of the camera set-up, before replacing the unreliable “Tarion” magic arms by the Aputure A10 arms.