ASTM E94 — Standard Guide for Radiographic Examination
General guide for radiographic examination — covers technique selection, equipment, materials, and procedure development for industrial RT.
Scope
ASTM E94 — Standard Guide for Radiographic Examination Using Industrial Radiographic Film — is the foundational ASTM general guide for radiographic examination. While not a directly invoking acceptance standard, E94 provides the technical framework for industrial radiography that other ASTM and ASME standards build upon. It covers source selection, source-to-film distance, IQI selection, film selection (speed and contrast classes), processing, and viewing — applicable across welded steel, castings, and base materials. ASTM E94 is referenced by ASME Section V Article 2 as SE-94, by ASTM E165 (weld RT for non-ferrous), by E1742 (radiologic interpretation), and many specialized RT standards. The current published version is ASTM E94/E94M-17 (reapproved 2023). Coverage spans both X-ray (kilovoltage) and gamma-ray (Ir-192, Co-60, Se-75) sources.
Code compliance is only demonstrable if the evidence behind it is: the procedure revision in force, the inspector's certification state and the instrument's calibration status at the time of test. Atlantis NDT provides ASNT Level III consulting for procedure and written-practice work against this code, training toward the certifications that reference it, and inspection management software that keeps that evidence recoverable years later. Request a consultation.
How a standard like this is applied in an inspection programme
A standard is only half of the requirement. It defines how an examination is performed and, in some cases, how results are classified — but the acceptance criteria that decide whether a component stays in service normally come from the construction or in-service code governing the item, not from the examination standard itself. Confusing the two is one of the more common findings in a procedure review: a procedure that correctly cites the examination standard but applies acceptance criteria from the wrong code or the wrong edition.
What has to be in place for compliance to be demonstrable
A written procedure qualified against this standard for the specific materials, thickness ranges and geometries in scope — not a generic procedure covering everything
Personnel certified for the method and level under ASNT SNT-TC-1A, ANSI/ASNT CP-189, NAS 410 or ISO 9712, current on the date the examination was performed
Equipment, probes and reference standards in calibration on that date, with traceability to a national standard under ISO 17025
The applicable edition of the standard recorded against the examination, so historical work stays assessed under the edition then in force
Technique sheets under the same revision control as the procedure above them — the most frequently uncontrolled document in an otherwise compliant quality system
Edition changes
When a new edition is issued, new work moves to it from a defined effective date that you set and record; work already performed stays assessed under the edition in force at the time. Retrospectively applying a new edition to historical dispositions invalidates the original acceptance decision and creates a substantially larger problem than the one being solved.
Where this usually goes wrong
Not in the technical content, but in reconstruction. An auditor picks an issued report and asks which procedure revision applied, who performed the work and whether they were qualified on that date, and whether the instrument and reference blocks were in calibration. Programmes that hold only current state can answer none of those. Binding the document revision, the qualification state and the calibration state to each inspection record as it is created turns that from an investigation into a lookup.
ASTM E94 is a guide, not a specification: it recommends film radiography practice and contains no acceptance criteria. It covers film system class, screens, exposure geometry, IQI selection and placement, backscatter checks, processing and storage, and caps geometric unsharpness at 0.020 in. below 2 in. of thickness. Density limits and accept or reject decisions come from the referencing code, never from E94 itself.
The single most consequential fact about E94 is grammatical. ASTM publishes standards in four voices, and a Guide uses should and may rather than shall. That means a purchase order reading radiography shall be performed in accordance with ASTM E94 has imposed almost nothing enforceable, because the document it invokes recommends rather than requires. The mandatory-language companion is ASTM E1742, the Standard Practice for Radiographic Examination, and inside the ASME world it is Section V Article 2. E94 remains valuable as the technical reference behind both: it is where the geometric unsharpness table, the film system classification logic, the backscatter check and the shim and IQI placement conventions are explained rather than merely stated. Read it as the engineering rationale, then write your technique sheet against the practice or the code that your contract actually enforces, and make sure the two agree line by line.
Source: ASTM E94/E94M, Standard Guide for Radiographic Examination Using Industrial Radiographic Film; ASTM E1742/E1742M, Standard Practice for Radiographic Examination; ASTM E1025 (hole-type IQI), E747 (wire IQI), E1815 (film system classification), E1079 (densitometer calibration), E1032 (radiographic examination of weldments), E2033 (computed radiography), E2698 (digital detector arrays); ASME BPVC Section V Article 2; ASTM E390, E446, E186, E280 and E155 reference radiographs.
E94 parameters, the document that actually enforces each one, and the finding it generates
Parameter
Value in practice
Document that makes it mandatory
Recurring audit finding
Geometric unsharpness, thickness under 2 in.
0.020 in. maximum
Referencing code or the client spec that adopts the E94 table
Ug never calculated; source-to-film distance chosen from habit, not from Ug = F t / d
Geometric unsharpness, 2 in. through 3 in.
0.030 in. maximum
Same table, adopted by contract
Thickness taken as nominal wall, ignoring reinforcement and the far wall on a double-wall shot
Geometric unsharpness, 3 in. through 4 in., and above 4 in.
0.040 in. and 0.070 in. maximum
Same table, adopted by contract
Source size read off the camera nameplate rather than the current source certificate
Film density through the IQI, X-ray source
1.8 minimum, single-film viewing
ASME Section V Article 2; ASTM E1742
Density read at a convenient clear area instead of through the IQI body
Film density through the IQI, gamma source
2.0 minimum, single-film viewing
ASME Section V Article 2; ASTM E1742
Iridium shot accepted at 1.9 because the technician carried over the X-ray number
Maximum density, single or composite viewing
4.0 maximum
ASME Section V Article 2; ASTM E1742
Illuminator too weak to interpret 4.0, so the densest region is never actually read
Density variation across the area of interest
minus 15 % to plus 30 % of the density through the IQI
ASME Section V Article 2
Variation checked only at the IQI, never at the extremities of the area of interest
Densitometer verification
Interval not exceeding 90 days against a certified step wedge comparison film
ASME Section V Article 2; ASTM E1079
Comparison film itself past its certification date, so the verification proves nothing
The unsharpness values are E94's own table. Every density figure here is enforced by the referencing code or practice rather than by E94, which is exactly why invoking E94 alone leaves them unenforceable.
What E94 Covers, and the Four Things It Deliberately Does Not
ASTM E94 is a guide to radiographic examination using industrial radiographic film, and the words industrial radiographic film in the title carry the entire scope argument. Within that boundary it is thorough: selection of X-ray energy or gamma source, film system class under E1815, lead screens and their thickness, exposure geometry and source-to-film distance, image quality indicators, backscatter protection, processing whether manual or automatic, viewing conditions, and storage of the finished radiographs. It is the document that explains why the numbers are what they are.
Four exclusions decide most disputes. E94 sets no acceptance criteria, so nothing in it can tell you whether a 0.09 in. rounded indication is rejectable. It does not qualify personnel; that job belongs to SNT-TC-1A, ANSI/ASNT CP-189, NAS 410 or ISO 9712 depending on the sector. It does not address radiation safety, which is regulatory rather than technical. And it does not cover computed radiography or digital detector arrays, which live in E2033, E2698, E2445 and E2446.
When a client specification says only radiography per ASTM E94, somebody still has to convert a guide into an enforceable technique sheet before the first exposure. Deciding source, energy, film class, IQI type and quality level, density window and coverage overlap, then defending those choices to a third-party reviewer, is exactly the work covered by ASNT Level III consulting.
Density: the 1.8, 2.0 and 4.0 Numbers and Where They Bite
Radiographic density is measured through the body of the hole-type IQI, or immediately adjacent to a wire IQI, and not at some convenient clear spot on the film. For single-film viewing the accepted minimum is 1.8 for radiographs made with an X-ray source and 2.0 for radiographs made with a gamma source, with a maximum of 4.0 for either single or composite viewing. Those values are enforced by ASME Section V Article 2 and by ASTM E1742; E94 supplies the reasoning behind them.
The variation rule catches more people than the limits themselves. Density anywhere within the area of interest must stay within minus 15 percent and plus 30 percent of the density measured through the IQI. On a thick-to-thin transition, a nozzle-to-shell weld, or a pipe shot at an angle, that band is genuinely tight, and it is the reason shims and multiple IQIs exist. A reviewer who checks density only at the IQI and never at the extremities of the area of interest has not performed the check.
Two practical traps follow. First, the densitometer itself must be verified at intervals not exceeding ninety days against a certified step wedge comparison film, and that comparison film carries its own expiry date. Second, a maximum density of 4.0 is meaningless if the viewing illuminator cannot deliver enough light to interpret 4.0, so the darkest region on an accepted film is frequently never really read. A radiograph that met the numbers on paper but was illegible in the viewing room has still failed the examination.
Geometric Unsharpness: the Table Everyone Quotes and Few Calculate
E94's unsharpness table is short enough to memorise: 0.020 in. maximum below 2 in. of material thickness, 0.030 in. from 2 in. through 3 in., 0.040 in. from 3 in. through 4 in., and 0.070 in. above 4 in. The formula behind it is Ug equals F times t divided by d, where F is the maximum projected source dimension, t is the distance from the source side of the specimen to the film, and d is the source-to-object distance.
Three inputs are routinely wrong. Source size is taken from the equipment nameplate rather than the current source certificate, and an aged iridium source in a replacement pigtail is not the size printed on the camera. Thickness t is taken as nominal wall when the real value includes weld reinforcement, backing, or the far wall in a double-wall double-image shot. And d is measured to the outside surface rather than to the source-side surface of the object. Each error pushes Ug the wrong way.
The audit consequence is specific and repeatable. A reviewer takes the shooting sheet, recomputes Ug from the source certificate, and finds the exposure exceeded the limit by a factor of two. Every radiograph made to that technique becomes suspect, not only the one examined. Technicians who can derive the geometry rather than copy last month's sheet are made, not born, which is what structured NDT training and certification to ASNT SNT-TC-1A exists to produce.
IQIs: Selection, Shims, Source Side and the Letter F
Hole-type IQIs are covered by ASTM E1025 and wire IQIs by E747. The quality level normally invoked is 2-2T, meaning an IQI whose thickness is two percent of the specimen thickness with the 2T hole visible on the radiograph. Selection is based on the thickness being radiographed, which for a weld means base metal plus the reinforcement actually present, not the thickness on the drawing.
IQIs are placed on the source side wherever geometry allows it, because that is where image quality is worst and therefore where the demonstration is honest. When the source side is inaccessible, film-side placement is permitted, and a lead letter F at least as high as the identification number must appear adjacent to or on the IQI so a reviewer knows which side it sat on. A film-side IQI without the F is a documentation nonconformance even when the image quality is excellent.
On welds, a hole-type IQI placed on the base metal beside the weld sits on less material than the weld itself, so a shim of radiographically similar material is placed under the IQI to bring the total up to the weld plus reinforcement. Omitting that shim makes the sensitivity demonstration easier than the examination it is supposed to certify, and it is one of the most frequently written findings on vendor radiography packages.
Backscatter, the Lead B, and the Half of the Rule People Forget
Scatter radiation arriving from behind the cassette degrades contrast without producing any recognisable artefact, so E94 prescribes a direct test rather than an opinion. A lead letter B, typically half an inch high and a sixteenth of an inch thick, is attached to the back of the cassette for each exposure, or at minimum for every setup where backscatter is credible.
The interpretation rule is asymmetric and is misquoted constantly. If a lighter image of the B appears on a darker background, backscatter protection was insufficient and the radiograph is rejected. If a darker image of the B appears on a lighter background, that is not cause for rejection. Technicians who reject on any visible B are scrapping sound film; technicians who ignore the B entirely are accepting radiographs whose contrast has been quietly eroded.
Backscatter is worst where it is least expected: small-diameter pipe shot inside a fabrication bay with a concrete floor and steel racking close behind the cassette, or field shots taken with the film pressed against a structural member. Adding lead sheet behind the cassette costs almost nothing. Re-shooting a completed weld package after a client's reviewer finds a light B on twenty films costs a great deal.
Acceptance Criteria Live Somewhere Else Entirely
Because E94 contains no accept or reject rules, every radiograph made under it is judged by a separate document. For pressure vessels that is ASME Section VIII Division 1, paragraph UW-51 for full radiography and UW-52 for spot radiography, which govern slag inclusion length and rounded indication charts. For process piping it is ASME B31.3, whose severity depends on the fluid service category. For structural steel it is AWS D1.1 Clause 8, and for pipeline girth welds API 1104.
Castings are a different world again. They are judged by visual comparison against reference radiographs: E446 for steel castings up to 2 in., E186 for heavy-walled castings from 2 in. to 4.5 in., E280 above that, E155 for aluminium and magnesium castings, and E390 for steel fusion welds. A specification that says radiography per E94 with no reference radiograph set and no severity level named has left the entire commercial argument on a casting order undefined.
The procurement trap is simple to state and expensive to hit. Radiography is technique plus acceptance, and a purchase order naming only the technique document invites the supplier to choose the most forgiving acceptance standard available. Name both, and name the revision of each. Keeping technique sheets, IQI selections, source certificates and acceptance references linked to the specific weld and the specific report is precisely what an inspection management system is for.
E94 Is the Wrong Document for Digital Radiography
Film radiography and digital radiography are not the same examination described in different words. Film has density, film class and screens. A digital detector array has grey values, signal-to-noise ratio, basic spatial resolution, bad pixel maps and a calibration state that drifts. There is no meaningful translation of a density of 2.0 into a digital image, which is why E94's scope statement names industrial radiographic film explicitly.
The correct documents are ASTM E2033 for computed radiography with imaging plates, ASTM E2698 for radiographic examination using digital detector arrays, and E2445 and E2446 for the qualification and long-term stability of those detectors. Duplex wire gauges under E2002 carry much of the burden a film technician would have discharged through density and IQI sensitivity alone, and ASME Section V has added dedicated digital articles for the same reason.
This matters commercially because computed radiography and digital detector arrays are now the default at many fabrication shops while legacy specifications have not caught up. A supplier shooting imaging plates against a specification that names E94 is working outside the standard it invoked, and a diligent client reviewer will say so at the least convenient moment. Auditing legacy specifications for exactly this mismatch, before the purchase order is placed, is routine ASNT Level III consulting work.
Writing a Procedure That Survives a Third-Party Review
A radiographic procedure written against E94 alone will fail review, because it inherits the guide's permissive language. The procedure must fix in writing every variable E94 leaves open: source or tube and its energy, film system class, screen material and thickness, source-to-film distance with the Ug calculation shown, IQI type, quality level and placement, density limits and where they are measured, processing method, and the coverage overlap between successive exposures.
The technique sheet then has to prove the procedure was followed on a specific joint. Reviewers look for the weld or component identification, an exposure geometry sketch, the number of exposures required for full coverage, the source certificate reference and its activity on the date of shooting, the IQI identification, the measured density values, and the interpreter's certification level and expiry. Missing source activity is the single most common gap, because it makes the exposure impossible to verify after the fact.
Where a package has to withstand an owner's or a certifying authority's scrutiny, an independent read of the films against the technique sheet is worth more than a second internal signature. That independent second look, performed on your existing packages and against the code you are actually contracted to, is what independent report validation delivers, and it is the fastest way to learn whether a radiography vendor's paperwork matches their film.
Digital radiography and E94: a guide, not a substitute for the digital standards
E94 was written for film radiography, and its guidance on geometric unsharpness, source-to-film distance and exposure technique transfers conceptually to digital radiography but is not itself the governing document once a computed radiography plate or a digital detector array replaces film. ASTM E2033 and E2698 for CR, and E2597 and E2737 for DDA, carry the parameters that actually control digital image quality — basic spatial resolution, contrast sensitivity, signal-to-noise ratio — and a procedure that cites E94 alone for digital work has cited the wrong family of documents for the technique actually being used.
This produces a specific and recurring audit finding: a shop transitions from film to CR or DDA, updates its equipment and its operators, but leaves the governing procedure referencing E94 because the underlying physics concepts feel unchanged. The procedure passes casual review because the geometry and exposure logic still reads correctly, and it fails a detailed audit because none of the digital-specific acceptance parameters — the ones that actually control whether the digital system is performing to specification — are addressed at all.
The practical fix is a procedure that names the correct standard for the detector technology actually in use, with E94 retained only for its still-relevant general radiographic principles rather than cited as the controlling document for image quality.
Is ASTM E94 a specification you can write into a purchase order?
Not usefully on its own. E94 is a Guide, which in ASTM's own hierarchy means it offers options and uses should rather than shall. A purchase order that invokes only E94 has bought you a recommendation. If you need enforceable radiography, invoke ASTM E1742, or ASME Section V Article 2 with the referencing Code Section, and cite E94 alongside as supporting technical background rather than as the governing requirement.
Does ASTM E94 contain acceptance criteria for weld defects?
No, and this is the most common misuse. E94 tells you how to make a readable radiograph, not what is rejectable on it. Acceptance comes from the construction code: ASME Section VIII Division 1 paragraphs UW-51 for full radiography and UW-52 for spot radiography, ASME B31.3 for process piping, AWS D1.1 Clause 8 for structural steel, or API 1104 for pipeline girth welds. Castings are judged against reference radiographs such as E446, E186, E280 or E155.
What geometric unsharpness does E94 allow at 1 in. wall thickness?
0.020 in., because 1 in. falls in the under-2-in. band of E94's unsharpness table. Calculate it as Ug = F t / d, where F is the source dimension, t is the distance from the source side of the specimen to the film, and d is the source-to-object distance. At 1 in. wall with a 3 mm source, that constrains source-to-film distance far more than most shooting sheets assume.
Why does the film density minimum change between X-ray and gamma?
Because gamma sources such as Ir-192 and Co-60 produce harder, less contrasty radiation, so a higher density is needed to recover equivalent radiographic contrast and sensitivity. That is why the accepted minimum through the IQI body is 1.8 for X-ray and 2.0 for gamma under single-film viewing. Technicians who switch from an X-ray crawler to an iridium shot mid-campaign and keep the same density target generate a genuine nonconformance.
Can ASTM E94 be cited for computed radiography or a digital detector array?
No. E94's scope is explicitly industrial radiographic film, and its film system classes, screen guidance and density limits have no meaning for a digital image described by grey value and signal-to-noise ratio. Computed radiography is covered by ASTM E2033, digital detector arrays by E2698 with detector qualification under E2445 and E2446. Citing E94 on a computed radiography job is an immediate procedure-review finding.
What does the lead letter B on the back of the cassette prove?
It is the backscatter check. A lead B, typically half an inch high and a sixteenth of an inch thick, is attached to the rear of the cassette. If a lighter image of that B appears on a darker background of the processed radiograph, backscatter protection was inadequate and the radiograph is rejected. A darker image of the B on a lighter background is not cause for rejection, which is the half of the rule most technicians forget.
Frequently asked
Is ASTM E94 still current, or has it been withdrawn?
It is current. Over successive revisions the title was narrowed to name industrial radiographic film explicitly, which tightened the apparent scope even though the technical content stayed broadly stable. Always cite the revision in your specification, because a document written a decade ago may have invoked a version whose scope statement did not exclude digital methods as clearly as the current one does.
What is the difference between ASTM E94 and ASTM E1742?
E94 is a Guide and uses should; E1742 is a Practice and uses shall. E1742 sets the mandatory requirements for making, processing, viewing and documenting film radiographs, including density, IQI and record requirements. If your contract needs enforceable radiography from an ASTM document rather than from ASME Section V, invoke E1742 and treat E94 as the technical rationale sitting behind it.
Do I need both a hole-type IQI and a wire IQI on the same shot?
Normally no. The referencing code or client specification names one system, hole-type under E1025 or wire under E747, together with the corresponding sensitivity requirement. Problems arise on multinational projects where a European specification calls wire IQIs to ISO 19232 while an American one calls hole-type, and the same weld gets radiographed twice. Resolve that conflict in the procedure, before the first exposure.
Does E94 tell me how many exposures a pipe girth weld needs?
It gives the geometric basis rather than a fixed count. The number of exposures follows from diameter, wall thickness, source size, source-to-film distance and the unsharpness limit, plus the coverage overlap your procedure fixes. Panoramic, double-wall single-image and double-wall double-image techniques each produce different counts, and the referencing code often dictates which technique is permitted at a given diameter.
Who is allowed to interpret radiographs made under E94?
E94 does not decide this. Interpretation authority comes from the employer's written practice under SNT-TC-1A or ANSI/ASNT CP-189, from NAS 410 in aerospace, or from ISO 9712 certification, and the referencing code states which of those is acceptable. In practice a Level II interprets and a Level III owns the procedure, the technique qualification and final technical authority for the method.