‘Full-spectrum’ Olympus E-M1

Spectral sensitivity of a converted mirrorless digital camera

equipment
cameras
Author

Pedro J. Aphalo

Published

2018-02-24

Modified

2023-04-21

Keywords

cameras, ultraviolet, spectra

Introduction

As a follow-up to my tests of objectives for UV-photography I decided to compare the spectral responsiveness of an off-the-shelf Olympus E-M1 mirrorless camera against an identical camera converted to full spectrum by replacement of the sensor’s infra-red blocking filter with UV-transmitting optical glass.

The present tests aim at assessing the change in sensor sensitivity at different wavelengths, from UVA to IR. The testing approach used here is only semi-quantitative.

Methods

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 used a Soligor 35 mm f:3.5 objective. This is a vintage objective of the type described as being based on the same optical design as the Petri Kuribayashi “Kuri”, Kyoei, Acall and similar (described by Savazzi n.d.). The barrels are different. The Soligor objective has an M42 mount, and was mounted on the E-M1 with a Fotasy M42 to MFT adapter with focusing helicoid.

I used different filters: two short pass UV transmitting filters and two long pass UV-absorbing filters (Table 1). To speed-up testing I used Manfrotto’s Xume magnetic filter attachment rings.


Table 1. Description of filters used.


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
zwb1 Purshee ZWB1 30 mm Purshee 2018

I used 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 2). 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 2. 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.


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 spectrometer or a spectrograph. On the other hand, using light sources and a camera allows testing the “camera + objective + filter” performance which is of interest.

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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.


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 heat sinks (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 cameras were 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.

Each camera had a Arca-Swiss type plate affixed at its base, and a matching clamp on a macro-focusing rail allowed quick swapping of cameras retaining a consistent position with respect to LEDs and target.


Figure 3. Photograph of the camera set-up, before replacing the unreliable “Tarion” magic arms by the Aputure A10 arms.


photo of camera rig

photo of camera rig

Test procedure

All objectives were set at f:8.0 for the test shots in the hope of reducing possible vignetting. Sensitivity was set to ISO 200. All images were saved as raw (.ORF) files. Paper slips with notes (Yellow Posit Notes, which fluoresce much less than white paper) were photographed to keep track of filters, and adapted objectives.

All measurements were done in a darkened room, but the fluorescence of some objects within the room could not be completely avoided. The noise floor of the camera sensor when 1/3 to 1/10 of the image area was averaged was extremely stable and close to 0.50, even for 20 s exposure. The reason for this was the use of black level subtraction in RawDigger. The value of 0.5 was subtracted from all readings.

The readings for ambient light in the darkened room were in all sessions less than 1.0, and in most cases between 0.5 and 0.7. The full scale for E-M1 raw .ORF files is 16384 (14 bits), but highest average readings in these tests were close to 2500 and overexposure affected no pixels in the target. This gives a “space” of over 10 EV for the test.

Extraction of data from RAW images

The images were analysed with RawDigger Research Edition version 1.2.23 (https://www.rawdigger.com/). Reading the R, G1, B, and G2 counts from the image area corresponding to the PTFE target, but excluding the possibly dirty border.

Several images were taken in duplicate, and repeatability of actual readings on the same area of the target was within +-1%, except when readings were small in which case the absolute errors were between 0.1 and 0.05. Sampling different regions of the target had also little effect on counts, again within +-1%.

About 1/2 of the measurements were done twice, on separate sessions, reassembling the testing set-up in-between. This resulted in differences in absolute counts of about +-5%, but this affected equally all the objectives tested in the session, and cancelled out by the use of the Soligor objective as reference for the comparisons.

Computations

All data were analysed and plotted in R, using packages from the tidyverse and r4photobiology suites. This report was generated with R and package knitr.

The first step consisted in obtaining correction factors for relative effective T for the different objectives in visible light. These factors were later applied when comparing the relative transmittance of the objectives in the UV and violet regions of the spectrum.

The almost identical data from the two green channels were averaged as the first step of calculations. When measurements were repeated in different sessions the values were averaged after correction of the small differences between sessions. The average values read for the target area in RawDigger were entered as raw data in R.

Results

Full-spectrum conversion

Results from comparison of two cameras.


Figure 3. Relative response of two digital cameras to radiation of different wavelengths. The horizontal dotted line highlights the response corresponding to no difference in response. “bg38.builtin” is a comparison of the converted camera with a BG38 filter attached against the built-in filter in the off-the-shelf camera.



Figure 4. Relative response of sensor channels in two digital cameras to radiation of different wavelengths. “bg38.builtin” is a comparison of the converted camera with a BG38 filter attached against the built-in filter in the off-the-shelf camera.


From the figures above we can infer that the built-in filter of the E-M1 has what looks like a very steep cut-off at around 370–380 nm, and that it has a very strong IR filter. More measurements are needed, but based on these data I did some comparison between the cameras.

Tests in daylight with different UV filters

Some preliminary shots showed that the off-the-shelf E-M1 produced images with the typical look of UV images using only a Heliopan UG1 (Schott glass) filter. Apparently not only the off-the-shelf camera is responsive to UV radiation, but the built-in IR filter is strong enough to reject NIR well enough. This needs further testing but earlier tests in daylight with the StraightedgeU filter showed little difference in exposure between the two cameras. In contrast with a Baader U filter the off-the-shelf E-M1 needed increased exposure.

Test with bare-bulb flash

This test only gives an idea of the differences in false colour. Auto exposure in TTL mode was used. A Godox AD200 flash, H200J head with a xenon tube (XenonFlashTubes.com). The head was connected with an EC200 extension to the main unit and had an AD-S2 aluminium reflector attached. On the camera a Godox Xpro-O transmitter was used to control the flash.

Indoors, the cameras were mounted on a tripod and the flash handheld. Cameras were tethered to a computer using Olympus Capture. Focusing was done manually with the help of a Convoy 2+ UVA flashlight with the zoomed in light live on a 23 inch monitor. Recently picked flowers were used as subjects. Outdoors both cameras and flash were handheld, and single auto focus mode was used.

The first obvious difference in the images is the more subdued or absent false colour after white balancing the images. The effect depended on the plant species. Autofocus did work in both cameras, but in the off-the-shelf camera with the Baader U filter, it was sluggish, even in full sunlight. With the Baader U filter it was necessary to use a higher ISO or to open the diaphragm in the off-the-shelf camera, with image quality deteriorating for darker subjects.

Equipment suppliers

Formatt Hitech: (https://www.formatt-hitech.com/) Baader Planetarium: (http://www.baader-planetarium.com/en/) Tiffen: (https://tiffen.com/tiffen-filters/) UVR Optics: (http://www.uvroptics.com) Sunwayfoto: (http://www.sunwayfoto.com/e_index.aspx) iShoot: (http://www.photoloving.com/) Aputure: (https://www.aputure.com/) Sirui: (http://www.sirui.eu/en/) Giottos: (http://www.giottos.com/) Mengs: (https://www.mengsphoto.com/) LED Engin: (http://www.ledengin.com/) Marktech: (http://www.marktechopto.com/) Nichia: (http://www.nichia.co.jp/en/) RECOM: (https://www.recom-power.com/) Ohmite: (http://www.ohmite.com) Bourns: (http://www.bourns.com/) Olympus: (https://www.olympus-europa.com/site/en/c/cameras/) Sigma: (https://www.sigmaphoto.com/) DSLR Astro TEC: (http://www.dslr-astrotec.de/index-eng.html) GFG Plastics: (http://gfgplastics.uk/)

Electronic components were sourced from distributors: Digi-key electronics: (https://www.digikey.com/en/) Mouser electronics: (https://eu.mouser.com/) Farnell: (http://www.farnell.com/) LUMITRONIX LED Technik GmbH: (https://www.leds.de)