Welcome to the board!
There seem to be at least two commonly used bulb code systems (perhaps my observations are specific to the USA).
In Europe and much of the rest of the world, there's one official system (the designations prescribed in the UN Regulations, formerly called the "ECE Regulations") universally used. Those designations are more or less a subset of designations promulgated by the IEC, that is International Electrotechnical Commission. Not all IEC types are adopted in the UN Regulations. Some of the UN bulb designations have replaced older designations. For example, W5W used to be W10/5, T4W used to be T8/4, P21W used to be P25-1, and P21/5W used to be P25-2.
There are many different bulb designation systems in use in North America. There are designations put forth by NHTSA, by ANSI, and by industry.
Industry designation examples include 168, 194, 1157, 3157, 7440.
Some bulbs have more than one designation. For example:
HB1 (NHTSA designation), 9004 (ANSI designation)
HB3 (NHTSA designation, also adopted in the UN regs), 9005 (ANSI designation)
Some bulbs have only an ANSI designation. Not a whole lot of examples, but one that comes to mind is the 9500 (an HID headlamp light source made by Sylvania, pre-Osram, and used only in the Lincoln Mark 8 of the 1990s)
In Japan, the situation is mixed: they use a lot of UN-designated bulbs, and have for many years, even before they began formally applying UN Regulations on lighting in the early 2000s. But they also have a fair number of bulbs with Japanese industry designations. Some of those are IEC designations, and some, who knows where they came up with them. Examples: D6, IH01.
I understand that W5W does not necessarily correspond to exactly one physical product but a category or family with some variability in the actual bulbs that meet that spec. For example they could be between 40 and 60 lumens.
It's not quite clear to me whether you have quite snapped to what this means. W5W is a category (type), but that doesn't mean it's a family. There's no such thing as "No, this is a 40-lumen W5W, I need a 60-lumen W5W". A W5W has an output spec of 50 lumens, +/- 20%. So yes, that's 40 to 60 lumens, but a sample that produces 40 lumens and a sample that produces 60 lumens are legally equivalent; they are both W5W bulbs (assuming they meet all the other specs). That doesn't preclude a maker from producing different varieties of W5W (standard, long life, heavy duty, etc).
Then there are these fully numeric codes such as 2825. As @-Virgil- basically says above, I guess this is an Osram/Sylvania number.
Right. Everywhere in the world, every maker has its own item number for whatever kind of bulb. It's common in America (but not universal) for the industry number to be the order number/part number, sometimes with additional characters denoting packaging variants, like you might see "194NABP2" meaning a blister pack of two 194NA bulbs. Speaking of that, there's no formal difference between "A" and "NA". It used to commonly denote the difference between bulbs painted amber (the ones that looked like they got hit with overspray when a taxicab or school bus was being painted) like "1157A" and bulbs made of amber glass, so-called natural amber, like "1157NA". But the NA bulbs had much better colorfastness (paint was easy to chip/scrape off, and lower grades of it burned off) so they were hugely dominant until concerns arose with cadmium in the glass, which pushed a return to coated bulbs, only this time with transparent amber coatings, and now Sylvania (for sure, and maybe others) use "A" for coated bulbs and "NA" for amber glass...except when they don't.
So how do you order, say, a standard 12-volt P21/5W bulb? Well, that's a Philips 12499, or an Osram 7528, or a Narva 17916, or a Wagner 17916, or a Lucas 380, etc.
What's a Philips 12-volt H4? Depends which one! A standard one is a 12342. A heavy-duty one is 12342/99. A French-spec one (yellow glass cover) was 12342/86. A +30 is 12342PR. A +50 is 12342HV. A +80 is 12342XP. A +130 is 12342XV. A blue "Crystal Vision" is 12342CV.
It works the same way in America; including the OEM and aftermarket product offerings, Osram-Sylvania offers about 10 different 9005 bulbs, differentiated by different letters after the "9005".
There are also types with numbers of unclear origin. 3496 and 3497 are examples, along with 3652 (and probably more the longer I sit here and think about it).
There are also "phantom" type numbers that have been whipped up into existence, out of thin air. For example, the "9008" pseudo-designation for what everybody in the world calls an H13 bulb. That "9008" looks like an ANSI designation, but there is no such ANSI designation. It doesn't exist. Some yahoo somewhere went "Well, huh, this bulb came after the 9007, so I guess it's a 9008!" and printed it in a manual somewhere, which caused people to go looking for a "9008" bulb, which caused makers to have to put "9008" alongside the real (H13) designation on the packaging. There's no end of noise that gets added to the designation systems by the less reputable ends of the aftermarket gutter, where you can learn all kinds of untrue baloney like "Well, there are two completely different kinds of H16 bulb, they look totally different and they don't interchange but they're both called H16".
And there's one guy in America who hand-gathers the bulb application info for most of the makers' application guides (or at least that used to be the case; I don't know if he's still at it), and he did it in a very low-tech manner: he went to car dealers and asked to see the bulbs for (or in) each kind of lamp in each model of car. When he was faced with a type he hadn't seen, he just faked it. That's how the 7440 bulb got its not-real alternate designation of "992", and it's how the PSX24W bulb got its fake designations "5202" and "2504". Those were batch codes and/or date codes on the unfamiliar bulbs, and this one Joe Schmoe elevated them into "designations" by putting them in application catalogs.
What about 168? Is that a Philips number?
It's a US industry number -- many, many years old.
Where can I find the specification that defines each bulb code within that system (e.g. shape, base, wattage, lumens, and stuff like that)? I know the answer may simply be to look at the spec sheet or web page for each individual bulb from each manufacturer but am looking for something easier to browse all at once
Well, UN Regulations
37 (filament),
99 (HID) and
128 (LED) cover all the UN types and are available as free downloads.
SAE J2560 covers halogen bulbs and contains a nice table giving the various designations for bulbs that have multiple designations (did you know H3 has an ANSI designation? It's 9200. Nobody ever calls an H3 bulb a 9200) as well as the specs for each type.
SAE J573 covers signal and marking bulbs, and has a similar table.
(I notice SAE has put their prices up
again...jeez!)
The Japanese specs are harder to get a hold of. The document is JIS C7506, and it comes in 3 parts. The part with the specs is Part 1. Mostly it's just a copy of the UN Regs, but with some additional data sheets for Japan-specific types you aren't very likely to run into.
Beyond that, it's hunt and peck. Donsbulbs is (sadly) dead and gone, which makes life harder when you're trying to find info like this. Dan Stern has (or used to have) a nice spreadsheet of the plastic wedge bulbs...everybody knows about 3157 and the other common ones, but there are a whole lot more than that, some of which are obscure and interesting.
Does anyone know of a master table/cross-reference that includes all automotive bulbs and shows the ECE code along with codes from all other common systems and links them so that people can easily convert between systems?
That Volvo post you found probably comes closest, but keep in mind there really isn't a "conversion" like you seem to want. 194 and W3W are generally cross-compatible, but they are
not the same bulb. They put out about the same amount of light, and they fit in each other's sockets, but they aren't built to the same specs, they don't have the same filament type, and they don't have the same base -- the 194 has a W2.1x9.2d while the W3W has a W2.1x9.5d. Similar goes for 168 vs. W5W. And 1156 vs. P21W, 1157 vs. P21/5W, 1893 vs. T4W, 67 vs. R10W, etc. And things get more complicated, in more detail, if we talk about the significant optical differences between HB2/9003 vs. H4. Fit and function cross-compatible, at least at the level most people care about and can see, but technically-legally-optically not the same bulb.
And speaking of different bases for very similar bulbs: even when the base (cap)/socket is the same, its designation can differ.
IEC (+UN, +Japan) says "BA15s", US says "SC Bayonet".
IEC says "BA15d", US says "DC Bayonet".
IEC says "BAY15d", US says "DC Bayonet Index".
IEC says "BA9s", US says "SC Mini Bayonet".
and so on, where "SC" = "single contact" and "DC" = "double contact".
The IEC cap/socket designations are pretty easy to decode at least in rough:
First character = general type. "BA" for bayonet, "P" for prefocus, "E" for Edison (screw), "W" for wedge, and a few others. Optionally followed by a letter signifying an indexing/keying variant within a family.
Second one or two characters = cap diameter in millimeters, except in case of Wedge bases where you have two sets of numbers defining the width and height of the base wedge.
Next character = number of contacts, "s" for single, "d" for double, "t" for triple, "q" for quadruple.
Optional suffix number for specifying a secondary diameter
BA15s: Bayonet base, 15mm diameter, single contact.
(variants: BAU15s and BAZ15s with index pins in different locations to prevent interchangeability of different-color bulbs)
BAY15d: Bayonet base with indexing/keying feature, 15mm diameter, double contact.
W3.1x16q: Wedge base, 3.1 x 16 mm, four contacts.
E26: 26mm Edison screw base, the common American household screw-type light bulb
E27: 27mm Edison screw base, the common European household screw-type light bulb
P43t-38: Prefocus, 43mm diameter, three contacts, with a 38mm diameter elsewhere on the base
(variants: PU435, PZ43t...)
PK22s: Prefocus, keyed/indexed, 22mm diameter, single contact
P20d: Prefocus, 20mm diameter, double contact (HB3/9005 bulb)
PX20d: Prefocus, key/index variant, 20mm diameter, double contact (HIR1/9011 bulb)
PZ20d: Prefocus, key/index variant, 20mm diameter, double contact (H12/9055 bulb)
P29t: Prefocus, 29mm diameter, three contacts (HB1/9004 bulb)
PX29t: Prefocus, index/key variant, 29mm diameter, 3 contacts (HB5/9007 bulb)
And so on.