Measuring campaign in the Alps

I joined a field measuring campaign organized by my collaborator T. Matthew Robson with the participation of José Ignacio García Plazaola and Beatriz Fernández-Marín from the University of the Basque-Country (see Matt’s CanSEE and my SenPEP blogs for information on our research). We spent the last week of May the at 2100 m a.s.l. in the Alps at the Jardin Botanique du Lautaret measuring solar radiation and the responses of plants to it. I did some measurements of solar radiation but spent most of the time photographing plants and lichens to record their optical properties in the ultraviolet-A, visible and near-infrared regions of the spectrum.

Each gallery can be viewed within this post or the at the Flickr website where all the photographs are stored. The thumbnail above each gallery is linked to the same gallery at Flickr.

The location and the crew

This first gallery contains some views of the site, an experiment and the crew (except myself).


I took photographs both outdoors under natural light and indoors using artificial light from LEDs or a flash. For some of the photographs I used focus bracketing and stacking to increase the depth of field.

The Station Alpine Joseph Fourier keeps an extensive set of excellent photographs of alpine plants from around the world at Flickr. They also have grouped the albums from the Alps into collections based on regions.

UV-A and VIS photograph pairs

Photographs taken indoors, in a dark room. Merged focus-bracketed stacks.

Photographs taken outdoors, either with sunlight or electronic flash as illumination.

Tusilago farfara L.

Gentiana acaulis L.

Pulsatila spp. (2 different species)

Crocus vernus (L.) Hill

Draba aizoides (??)

Potentilla crantzii(??)

Soldanella alpina

Primula (4 species)


UV-reflective leaf pubescence


The next galleries show lichens (at least Xanthoria (orange fluorescence from Parietina), Acarospora y Rhizocarpon (both with yellow fluorescence from rizocarpic acid). Almost all the photographed lichens were growing within a few square meters on a memorial of the Scott’s Antarctic expedition, erected within the botanic garden in 1913.

I took the first set of photographs using an Olympus E-M1 camera in sunlight. These photographs have a reliable white-balance from a reference white or grey card.

Images below taken with a full-spectrum converted Olympus E-M1 camera, using filters passing visible light, ultraviolet-A radiation (Baader U or StraightEdgeU) or near-infrared radiation (Hoya R72 or Zomei 850 nm). Visible light images colour was edited to match that seen with the help of a custom colour profile and reference images taken with an unconverted Olympus E-M1 camera (gallery above).

The size of the reference Teflon slab used is 100 mm x 100 mm. In the case of close-up photographs an automatic Kenko MFT macro extension tube with 10 mm length was used.

Lichens photographed on a moonless night with an of-the-shelf camera with a stack of UVA-blocking filters on the lens: Firecrest UV400 plus Tiffen Haze 2A. UVA light source: Convoy 2+ flashlight with Nichia 365 nm LED filtered with VIS blocking UVA-pass filter. Visible light source: Sunwayfoto FL-96 LED CRI 95+ set at 5000K.


UV and NIR photographs

An Olympus E-M1 camera converted to full spectrum by replacement of the sensor filter, a Sigma 30mm f:2.8 DC A objective and selective filters were used. The transmission spectra of two of the filters used are shown below.

Spectral transmittance of an ultraviolet band-pass Baader U-filter.
Spectral transmittance of a Hoya R72 IR long-pass filter

UV-induced visible fluorescence

The emission spectrum of the filtered UV-A flashlight used for excitation is narrow, and contains almost no visible light.

Spectral irradiance at the center of the beam at a distance of c. 28 cm.

The filter stack blocks the ultraviolet radiation from the flashlight (cf. spectrum above) but transmits the fluorescence emitted at longer wavelengths. The internal filter of the unmodified Olympus E-M1 camera blocks most radiation longer than 700 nm and shorter than 380 nm.

Spectral transmittance of the UV-blocking filter stack


Most neutral density filters are not neutral

A neutral density (ND) filter is a “grey” filter, a filter that transmits equal fractions of the incident radiation at all wavelengths. A perfectly neutral filter over a broad range of wavelengths is an idealized concept, and one very difficult to implement in practice. There are different approaches to making filters approximating colour neutrality. We here compare the spectral transmittance of of ND filters of three different types available for use on camera lenses and explain why the use of some of them can introduce strong colour casts in the photographs we take with them.

Continue reading Most neutral density filters are not neutral

Capture One to the rescue

Rescuing a technically “bad photograph” is not that difficult nowadays. This photograph was taken against all odds… through the double glassing of a dirty window on a train racing at high speed through the landscape. To make things even worse the sun was shining on the window I took the photograph through and the glass was slightly tinted green. The result out of camera was a low contrast raw image that looked like a sure discard… but was it?

Default rendering of the RAW file in Capture One 12.

Continue reading Capture One to the rescue

How small a spectrometer can be made?

[I will update this post after testing the sensor]

In a recent post I described a miniature two-channel UV-A sensor with digital interface. Here I will describe a miniature and low cost spectrometer, type AS7265X from ams. It does not used a grating as monochromator, but instead each of the 18 channels has a different interference filter deposited directly on the silicon chip. The FWHM is 20 nm, and the wavelength range from 410 nm to 940 nm. The spectrometer consists in three separate sensor units working together. The interface is digital, and temperature compensation and analogue to digital conversion takes place in the sensor modules. In spite of the number of channels communication between the spectrometer and a micro-processor requires only two wires. The spectrometer supports two different communication protocols, the specialized I2C and a generic serial communication (UART).

Macro-photographs of both sides of an early prototype of a breakout board are shown below. The size of the board is 18 mm × 19 mm. (Photographs were taken as described for the UV-A sensor.)

I bought this board from a seller at Tindie for USD 50. The seller is now selling a differently shaped board, with the three modules in a triangle, and so closer to each other.

How small can a UV-B plus UV-A sensor be made?

[updated 2019-02-13] [I will update this post again after testing the sensor]

Rather recently Vishay announced a miniature sensor under the name VEML6075 with two channels nominally centred at 365 and 330 nm. The peak width at half maximum is 20 nm. So, in practice it is a sensor measuring two regions within the UV-A band with the tail of one of the two channels extending into the UV-B. It is not a sensor capable of separately measuring the UV-B and UV-A bands. However, under sunlight it collects enough information to obtain a reasonable estimate for the UV Index (see the application note from Vishay for deatils).

It is not just a sensor but instead a sensor module with a digital interface. It has all the electronics for temperature compensation and for converting the analogue signals from the sensor into digital data with a rough calibration applied. The package of this sensor is 1 mm thick and 2 mm times 1.25 mm in area. The sensor itself is much smaller and it follows reasonably well the cosine law without any diffuser. Price? Less than 2€ as a component… and between 4 and 7 € for a breakout board.

A breakout board is a small printed circuit board usually containing a single, or very few components. The components included are only those needed for a single complex integrated circuit or sensor module to function, and given the small size of the components, the board allows easier soldering by hand of wires. I have bought two different breakout boards with the same VEML6075 sensor. They differ in size, the smaller boards has components on both sides, while the larger one only on one side. (Drag the slider to see the bottom of the boards.)

The images above cover an area of 23 mm × 17 mm. I took a pair of photographs at higher magnification, and as it fits a UV sensor, I photographed it both in visible light and in UV-A radiation. The whole image is 3 mm tall by 4 mm wide.

Technical information about the photographs

All photographs were taken with an Olympus E-M1 digital mirrorless camera, tethered to a laptop computer and controlled using Olympus Capture 2 software.  A camera converted to full spectrum was used.

For the images at lower magnification I used a modern M.Zuiko 60 mm f:2.8 Macro objective, a Sunwayfoto FL-96 LED light source. I took focus-bracketed stacks of between 15 and 35 images, depending on the depth of the electronic components on each side of the boards. I merged the stacks of raw images using Helicon Focus 7 and edited and converted the images to compressed JPEG format with Capture One 12.

For the higher magnification photographs I used a Zuiko 38mm f:3.5 macro objective (Olympus OM-System ca. 1972-1975+, single coated early version). The visible source was the same, and the UV-A light source was a Convoy 2+ 365nm UV-A flashlight filtered with a visible blocking filter. For the UV-A photographs a used on the objective a zwb1 2mm thick filter.

All the images are slightly cropped from the full frame, most to better align them for the slider. The photograph below shows the nearly 50-years-old objective. It is very small and its mount is the same as used for microscope objectives.

Zuiko Macro 38 mm f:3.5, RMS mount, Olympus OM-System ca. 1972-1976.

More on taking photographs through windows

The “Ultimate Lens Hood” seems like a good tool. It is still to be seen if it is stiff enough and/or a bit sticky so as to easily stay in place on the glass surface. It has the potential for being very useful but how easy it will be to handle with different lenses is still to be seen.

I just made my pledge for one ULH at Kickstarter. If you want to get your own, be aware that the campaign is about to end.

An oversized conical bellows of black silicone: the “Ultimate Lens Hood” (ULH).

See my earlier post to learn how I have been managing until now with normal collapsible lens hoods made of rubber. I recently uploaded a gallery of photos from an ongoing project where I am taking photographs through train windows.

Aputure’s Amaran AL-M9 vs. Sunway Foto’s FL-96

Aputure’s Amaran AL-M9 and Sunway Foto’s FL-96 are very small and handy LED light sources. The Amaran AL-M9 has been relegated to a second place in Aputure’s catalogue by the Amaran AL-MX, but I haven’t bought this newer and three times as expensive version. Both light sources compared here are roughly within the same price range.

Continue reading Aputure’s Amaran AL-M9 vs. Sunway Foto’s FL-96