[Updated 2019-07-17] 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.
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?
[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.
Visible vs. UV-A false-colour
Continue reading Photographs of flowers: VIS vs. UV-A
[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.
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.
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.
I have updated the post ‘Black anodised aluminium in IR‘ after using the Tenenal special camera paint (black deep mat) and repeating some of the tests.
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.
A comparison between the Baader U filter and the StraightedgeU filter, both with sun and a modified flash as light sources. Examples of flowers from two species, which display different false-colours with the two filters. Continue reading Filters for UV photography