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Lesson 15

Ilford EM10: Projected Light Measurement

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Welcome to the next stage of our journey into darkroom printing. In our last session, we established a systematic workflow for producing a 'work print'—a technically sound print that acts as a reliable foundation. You learned to "expose for the highlights and control the shadows with contrast," a method that, while effective, can involve several iterative test strips. Now, we'll explore a tool designed to streamline that process significantly, which I know is of particular interest to you: the Ilford EM10 exposure monitor.

This lesson focuses on the first step to mastering this device: understanding its operating principle. We won't just look at what it does, but how it does it. Given your engineering background, we'll go under the hood to examine the physics and electronics that allow this simple-looking tool to measure light with precision. This will build the foundation for the next lesson, where we'll calibrate it for practical use.

The Ilford EM10, a simple yet powerful photometer for darkroom printing.

What is the EM10's Purpose?

At its core, the Ilford EM10 is a specialized photometer. Its sole job is to measure the intensity of the light projected by your enlarger onto the baseboard. This allows you to achieve something crucial for an efficient workflow: consistency.

Instead of relying solely on visual assessment of test strips, the EM10 provides a quantitative target. Once you know the correct light level for a good print, you can use the meter to set that exact same light level for subsequent prints, even when changing print size or negatives.

Let's start by reviewing the manufacturer's description and seeing a quick demonstration.

[PDF] EM10 Instructions - Ilford Photo

This is the official instruction manual for the device. It provides a concise summary of its function.

Read the Introduction section. The key sentence to focus on is: "Basically, the EM 10 exposure monitor enables the intensity of the light falling on the baseboard to be easily set to a constant value." Notice how it works: you vary the lens aperture until the green LED is lit.

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The presenter, from the DistPhoto channel, gives a clear, practical demonstration of the EM10's user interface.

First, watch the brief introduction from the overview. Then, skip ahead and watch the demonstration from how it works. Pay attention to how the red and green LEDs guide the user. The red light on the left means "underexposed" (not enough light), the one on the right means "overexposed" (too much light), and the green light means you've hit the target light level set on the dial.

So, the EM10 is a null-seeking device. You set a target value with the dial, and you adjust the light (via the enlarger's aperture) until the meter tells you you've met that target. But what is actually happening inside the meter when it "sees" the light?

The Heart of the Meter: The Photoresistor

The sensing element in the EM10 is a component called a photoresistor, or Light-Dependent Resistor (LDR). For a component that enables so much control, its principle is elegantly simple: its electrical resistance changes depending on the intensity of the light that hits it.

The material used in most common photoresistors, including the one in the EM10, is Cadmium Sulfide (CdS). This is why you'll often see them referred to as "CdS cells." This material has a key property: its resistance is very high in the dark but drops significantly when exposed to light.

To understand why this happens, we need to look at the "internal photoelectric effect," a concept that should be familiar from your physics studies.

[PDF] Photoresistor Basics: Types, Principles and Applications

This article from Utmel provides a great technical overview of how photoresistors work, connecting the physics to the electronic behavior.

First, read the introductory paragraphs to get a feel for the terminology (LDR, photoconductor, dark vs. bright resistance). Next, dive into the core physics in the section on the working principle. Read from "Working principle" down to the end of that paragraph. This explains the internal photoelectric effect. Finally, read the section "Structural principle" for a reinforcement of the concept. It describes how photons generate electron-hole pairs, which act as charge carriers and lower the material's resistance.

In essence:

  1. In the dark, the semiconductor material (CdS) has very few free charge carriers (electrons and holes), so its resistance is extremely high (often in the mega-ohm range).
  2. When light from the enlarger strikes the CdS cell, photons transfer their energy to electrons in the material.
  3. This energy allows electrons to jump from the valence band to the conduction band, becoming free to move. This process also leaves behind a "hole," which also acts as a mobile positive charge carrier.
  4. The presence of these mobile electron-hole pairs drastically increases the material's conductivity, which means its resistance drops. More intense light creates more charge carriers, leading to lower resistance.

From Resistance to a Red or Green Light

So, the meter has a component whose resistance is a proxy for light intensity. How does this translate into the simple red/green LED display? The photoresistor is part of a larger electronic circuit. While the exact schematic for the EM10 isn't public, its function can be understood by looking at a representative circuit.

This is the circuit board from a simple garden solar light, but it illustrates the key components found in the EM10. We can see a photoresistor (labeled CDS), resistors (R1, R2, etc.), LEDs, and a variable resistor (VR).

The circuit in the EM10 works as a comparator.

  • The numbered dial on the EM10 is connected to a variable resistor (like VR in the image). When you turn the dial to, say, "20", you are setting a specific reference resistance within the circuit. This establishes a target voltage.
  • The photoresistor (the CDS cell) forms another part of the circuit, likely a voltage divider. The voltage across it changes as its resistance changes with the light from the enlarger.
  • A comparator circuit (likely built with transistors or a simple integrated circuit) continuously compares the voltage from the photoresistor leg of the circuit to the target voltage from the dial leg.
    • If the light is too dim, the CdS cell's resistance is high. The voltage comparison is out of balance in one direction, and the circuit lights up the "too low" red LED.
    • If the light is too bright, the CdS cell's resistance is low. The voltage is out of balance in the other direction, lighting the "too high" red LED.
    • When you adjust the enlarger aperture so the light intensity is just right, the CdS cell's resistance reaches a point where its voltage perfectly balances the target voltage from the dial. The comparator detects this "null" state and switches on the green LED.

You are, in effect, using the enlarger's aperture to physically "solve" an electrical equation inside the meter.

A Practical Complication: Color and Contrast

There is one final, crucial aspect of the EM10's operating principle. As we've discussed, multigrade papers work by responding to different colors of light (yellow/green vs. blue/violet) to change contrast. The CdS photoresistor also has its own spectral sensitivity—it doesn't respond equally to all colors of light.

This creates a slight disconnect. The change in the meter's reading when you switch from a Grade 2 (greenish) to a Grade 4 (magentish) filter might not perfectly correspond to the change in exposure the paper actually needs.

[PDF] EM10 Instructions - Ilford Photo

Let's return to the instruction manual to see how Ilford addresses this.

Read section "5 The use of the EM10 with black and white papers". The key takeaway is that for variable contrast papers, "a separate calibration will be required for each of the MULTIGRADE filters." If you don't do this, prints made at higher contrast will be too dark, and those at lower contrast will be too light.

This is not a flaw in the meter, but a direct consequence of its operating principle clashing with the physics of multigrade paper. It's a systematic offset that we can, and will, account for when we learn to calibrate and use the meter in our next lessons.

Conclusion

You now understand the fundamental principles behind how the Ilford EM10 works. It is not a magic box, but an application of solid-state physics and basic circuit design to solve a practical darkroom problem.

Key Takeaways:

  • The EM10 is a photometer that allows a printer to set the enlarger's light output to a consistent, repeatable intensity.
  • It operates using a photoresistor (LDR), typically a Cadmium Sulfide (CdS) cell, whose electrical resistance decreases as the intensity of incident light increases.
  • The physical mechanism is the internal photoelectric effect, where photons create mobile electron-hole pairs in the semiconductor, increasing its conductivity.
  • An internal comparator circuit balances the resistance from the CdS cell against a target value set by the user's dial, indicating a match with a green LED.
  • The meter's spectral sensitivity differs from that of multigrade paper, meaning a separate calibration is needed for different contrast filters to maintain exposure accuracy.

Next Lesson Preview:
Now that we've covered the theory of how the EM10 measures light, we'll move on to the practice. In the next lesson, you will learn how to calibrate the EM10. This involves using the work print you've already made to "teach" the meter what a correct exposure looks like for your specific enlarger, lens, and paper combination, turning it into a truly predictive and time-saving tool.