A densitometer is one of the most valuable tools a darkroom enthusiast can employ.  Up until recently, however, densitometers were expensive and bulky.  Furthermore, densitometers which would talk to your computer and calculate effective film speed, contrast index or gamma were beyond the reach of almost all amateurs.

 Easily programmed microprocessors and the availability of highly  accurate, inexpensive  photodiodes make it possible for the photographer to build a densitometer which is reliable, accurate and inexpensive.  The unit described here uses a new RGB photo-sensor, actually a 64 photodiode matrix with an incorporated light to frequency converter, the Texas Advanced Optical TCS-230  to greatly simplify the task. We chose to use the Basic Stamp II from Parallax Inc. as the microcontroller for this project since the device is relatively easy to program (it can be programmed right on the densitometer circuit board without having to be removed) .  If you ever get bored with the densitometer application for the Basic Stamp II  you can always press it into service in another device. An outline of the Computing Densitometer is shown below:

The outline above shows the several control lines from the Basic Stamp II to the TCS230 photo-receptor.  The TCS230 integrated circuit has five input lines: two for color measurement, two to control the TCS230 electronic iris, and  an Output Enable line (/OE).  There is one data line and, of course, power and ground lines.

 The design presented in this article uses the Basic Stamp II to  “talk” to your computer via its serial port, placing the data directly  into a Microsoft Excel spreadsheet. Parallax makes available free to users of the Basic Stamp an Excel Macro which accepts the data into your PC from  www.parallax.com.  The macro, StampDAQ can be downloaded from the Parallax website.  With the macro in place, you can plot the data and calculate information in real time for determining film and development parameters, or in later use while  printing.  While Excel can be used to convert the raw light readings into the logarithmic values which photographers are accustomed to using, we have incorporated a routine in the program which does so that log values can be seen on the LCD display.

 If there is enough interest we will modify the transmission densitometer to take reflection measurements.  It’s my opinion that transmission and reflection densitometers are, as Perry Como sung “like a horse and carriage, you can’t have one without the other!”

 The Sensor:

The TAOS TCS230 optical sensor consists of 64 individual photodiodes in a 4 by 16 matrix.  Light impinging on the sensor is converted to current which is, in turn, converted to a square wave whose frequency is proportionate to the intensity of the light.  A block diagram is shown below:

Since a white LED is used to illuminate the negative, the Hoya filter is unnecessary as most of the infrared spectrum is absent.  Texas Advanced Optical confirms this view.  The use of a “light-to-frequency” converter such as the TCS230 dispenses with the need for an expensive, high resolution analog to digital converter and its associated  low-noise voltage reference since the output of the TCS230 is  a series of pulses whose duration can be directly measured by the microcontroller.  The frequency of the pulses is as high as 500kHz  allowing the TCS230 to have far greater precision, on the order of 12 to 14 bits,  than would be available for a device using an ADC.  In addition, techniques can be employed to average the measurements, reducing the effects of random noise in measurements.

 The Circuit:

The subcircuits for the computer densitometer consists of an illumination circuit, and a measurement and communication circuit with LCD display panel.

 The illumination  consists of a White LED  powered by  a simple constant current source. In this case we use an inexpensive LM317LZ positive voltage regulator arranged as a current limiter.  With the 62 (61.9) ohm resistor shown,  20 milliamps will be drawn by the LED. The constant current source for the LED insures that brightness does not change over the course of a measurement session.   As mentioned previously, by using a white LED as illuminator very little radiation in the infrared portion of the spectrum is present. 

 The photo-receptor and measurement subcircuit consist of a the TCS230 light to frequency converter, the Basic Stamp II  microprocessor, an LCD display and associated switches, power supplies.  Power is derived from a common 9 volt “wall wart” transformer.  A schematic diagram for the computing densitometer can be seen by clicking on the thumbnail shown below:

Construction:

Illuminator: The illuminator circuit, consisting of the White LED and a constant current source. Current to the white LED is determined by measuring the voltage across resistor R3.   A Texas Instruments INA413 Differential Amplifier, set up for a gain of 10, is used for this purpose.  The voltage from the differential amplifier  is compared with the LM329DZ reference through operational amplifier U2.  While I have shown a self-calibrating precision CMOS opamp, the TLC4501 in this schematic, a less expensive device could also be used.    Operational amplifier U2 drives the base of the current source to the LED. 

Detector:  The TCS230 photodetector is placed on a small printed circuit board which fits into the head of an articulated arm. I used a small link of fiber-optic cable, cemented onto the case of the TCS230 so that device would not have to come into contact with the negative.  It is important that the detector be shielded from extraneous light and heat since infrared leaks will lead to inaccurate or non-repeatable measurements.  The arm is brought into contact with the negative to make a measurement when the “Measure” button is depressed.  I used CAT5, 8-conductor cable to make the connections from the TCS230 photo-receptor to the microcontroller. The topside PCB layout is shown below: