Vendor exposure indices: Philips EI, Siemens EXI, Fuji S, and the IEC standard

What exposure index is for: signal vs noise feedback

The image detector in digital radiography (DR) or computed radiography (CR) collects photons from the x-ray beam. The number of photons it collects determines the signal-to-noise ratio of the image. Too few photons, and noise dominates. Too many photons, and you’ve delivered unnecessary dose.

Exposure index is the detector’s feedback on how many photons it received. Think of it as the detector saying, “I collected this much radiation. Did that match your target, or did you over/under-expose me?”

The problem is that different manufacturers named this feedback differently, using different abbreviations and different scale directions. A technologist moving from a Siemens room to a Philips room needs to understand what each vendor calls the same concept, or the feedback becomes meaningless noise.

The common wrong answer (Siemens and Philips use the same abbreviation)

A common wrong answer on practice exams is: “Siemens and Philips both use EXI” or “Philips and Siemens both use EI.”

This is wrong.

• Siemens uses EXI (Exposure Index, Siemens proprietary name)
• Philips uses EI (Exposure Index, also happens to match the IEC 62494-1 standard abbreviation)

Both measure the same thing (exposure received by the detector), but they use different abbreviations. This confusion comes from the fact that EI and EXI sound like the same abbreviation, and both vendors are calling it “Exposure Index” in English. But on the display, one says “EXI” and the other says “EI.”

The ARRT does test this distinction, especially in scenario questions where you have to read a tech chart from a specific vendor and decide whether the actual value hit the target.

Vendor-by-vendor exposure-index table

Vendor	Abbreviation	Scale direction	Notes
Siemens	EXI	Higher = more	Higher EXI means more exposure. Target varies by protocol.
Philips	EI	Higher = more	Higher EI means more exposure. Philips also displays IEC 62494-1 standardized EI.
Fuji	S-number	Inverse (lower = more)	Fuji uses a sensitivity scale, not a direct exposure metric. Lower S-number means higher exposure.
Carestream/Kodak	EI	Higher = more	Same abbreviation as Philips, same direction. Carestream also displays IEC standard EI.
Agfa	lgM	Logarithmic	Log of the median scan value. Reflects the statistical midpoint of the histogram.
Konica Minolta	S-value	Inverse (lower = more)	Similar to Fuji: lower value indicates higher exposure, higher value indicates lower exposure.

IEC 62494-1: the unified exposure index standard

In 2008, the International Electrotechnical Commission published IEC 62494-1, defining a single unified exposure index (EI) that applies across all vendors. This was a major step toward dose optimization, because it meant you could finally compare exposure values from a Siemens machine to a Philips machine to a Fuji machine and know whether they were equivalent.

The IEC standard defines EI as the exposure that produces a specific signal level on a reference image. All vendors are now required to display this standardized EI alongside (or instead of) their proprietary vendor-specific values.

Modern DR and CR systems display both:

• The vendor-specific value (EXI for Siemens, EI for Philips, S-number for Fuji, etc.)
• The IEC 62494-1 standardized EI (same abbreviation across all vendors)

This dual display is why you might see both “EXI = 280” and “EI = 200” on a Siemens display. The first is Siemens’ proprietary scale; the second is the IEC-standardized value that lets you compare to other vendors.

DI (Deviation Index): comparing actual to target

Once you have an EI value, how do you know if it’s correct for the exam you just did?

Deviation Index (DI) answers that question. DI compares the actual EI (what the detector received) to the target EI (what you aimed for based on the protocol).

DI = log2(Actual EI / Target EI) × 10

Expressed in stops:

• DI = 0: You hit the target exactly. Perfect exposure.
• DI = +2: You delivered twice the target exposure (overexposure by 1 stop). 2x the dose, but the image might look the same visually.
• DI = -3: You delivered one-eighth the target exposure (underexposure by 3 stops). 1/8 the dose, but noise will dominate.

The ARRT expects you to interpret DI in the context of dose optimization. A DI close to 0 is the goal. Repeated positive DI values mean you’re over-exposing systematically and delivering unnecessary dose. Repeated negative DI values mean you’re under-exposing and might be delivering subdiagnostic images.

Why this matters: technique adjustment based on EI feedback

On any digital system, you adjust technique (kVp, mAs) to hit the target EI. Unlike film, where you had to guess, digital tells you whether you succeeded.

If you do a chest x-ray with technique calculated to produce an EI of 200, and the system shows EI = 400 (DI = +1), you overexposed by a factor of 2. On the next chest, you should reduce mAs by half to bring EI back to target.

But the abbreviation on the display matters for understanding what you’re reading. If you walk into a Siemens room expecting “EI” and see “EXI = 400” instead, you might panic or second-guess the value. Once you know that EXI is just Siemens’ name for the same concept, the panic goes away.

Why this matters on the ARRT

The ARRT Image Production section includes questions on exposure feedback in digital workflow. The test patterns include:

1. Vendor identification: “A Siemens DR system displays EXI = 250. What does EXI stand for?” Answer: Exposure Index (Siemens proprietary abbreviation).
2. Comparative questions: “Philips calls this value EI. What does Siemens call it?” Answer: EXI.
3. Scenario questions: “A facility uses both Philips and Siemens DR systems. The target EI is 200 on both. A Philips system shows EI = 180 (actual), a Siemens system shows EXI = 220. Which system is closer to target?” Answer: Siemens (EXI 220 is 20 above 200; Philips EI 180 is 20 below 200, but the direction might vary by protocol, the key is recognizing both are measuring the same thing).
4. DI calculation: “A Philips system shows actual EI = 400, target EI = 100. What is the approximate DI?” Answer: DI = log2(400/100) × 10 = log2(4) × 10 = 2 × 10 = +20 (or about +2 stops, meaning 4x overexposure).

The underlying principle is the same across all vendors: higher EI / EXI / S-number feedback means you’re delivering more dose and getting a better signal. Your job is to adjust technique to hit the target as close as possible (DI near 0) and understand that different vendors use different names for the same concept.

Quick reference table

Term	Definition	Abbreviation	Range
Exposure Index (IEC standard)	Measure of photons received by detector; standardized across vendors	EI	Varies by protocol
Siemens Exposure Index	Siemens proprietary exposure feedback	EXI	Varies by protocol
Philips Exposure Index	Philips proprietary exposure feedback (also matches IEC standard)	EI	Varies by protocol
Fuji Sensitivity Number	Fuji’s inverse-scale exposure feedback	S-number	Lower = more exposure
Deviation Index	Difference between actual and target EI, in stops	DI	0 = target; +/- values = over/under
Target EI	Protocol-defined desired exposure level	Target EI	Protocol-specific

ARRT exam tip

The ARRT does not expect you to memorize every vendor’s exact EI scale or to perform DI calculations under time pressure. What the exam does expect:

1. Know that different vendors use different abbreviations for exposure index.
2. Recognize that Siemens uses EXI, Philips uses EI, and Fuji uses S-number.
3. Understand that the IEC 62494-1 standard unified EI now appears on all modern systems.
4. Interpret DI values: positive DI = overexposure, negative DI = underexposure, DI near 0 = target hit.

If you see a question asking “What does EXI stand for?” the answer is “Exposure Index (Siemens)” or just “Siemens exposure index.” If it asks “What is the difference between EI and EXI?” the answer is “Different vendor abbreviations for the same concept, Philips uses EI, Siemens uses EXI.”

For a deeper look at the full digital workflow and how exposure index fits into image production, see our chapter on image acquisition and technique. For step-by-step technique adjustment based on exposure feedback, see computed radiography and digital workflow.