diff --git a/src/filter-parse-async.ts b/src/filter-parse-async.ts
index b31e421..02141c6 100644
--- a/src/filter-parse-async.ts
+++ b/src/filter-parse-async.ts
@@ -7,7 +7,7 @@ interface BitmapInfo {
   height: number
   interlace: boolean
   bpp: number
-  depth: number
+  depth: 1 | 2 | 4 | 8 | 16
 }
 
 /**
diff --git a/src/filter-parse-sync.ts b/src/filter-parse-sync.ts
index 0296f06..c1348d8 100644
--- a/src/filter-parse-sync.ts
+++ b/src/filter-parse-sync.ts
@@ -7,7 +7,7 @@ interface BitmapInfo {
   height: number
   interlace: boolean
   bpp: number
-  depth: number
+  depth: 1 | 2 | 4 | 8 | 16
 }
 
 /**
diff --git a/src/filter-parse.ts b/src/filter-parse.ts
index af34b49..aabc562 100644
--- a/src/filter-parse.ts
+++ b/src/filter-parse.ts
@@ -7,7 +7,7 @@ interface BitmapInfo {
   height: number
   interlace: boolean
   bpp: number
-  depth: number
+  depth: 1 | 2 | 4 | 8 | 16
 }
 
 interface FilterDependencies {
diff --git a/src/parser-async.ts b/src/parser-async.ts
index 5f0d0d5..a5ce6a6 100644
--- a/src/parser-async.ts
+++ b/src/parser-async.ts
@@ -14,7 +14,7 @@ interface ParserOptions {
 interface BitmapInfo {
   width: number
   height: number
-  depth: number
+  depth: 1 | 2 | 4 | 8 | 16
   bpp: number
   interlace: boolean
   palette?: Buffer
diff --git a/src/png.ts b/src/png.ts
index afbebeb..249b0d9 100644
--- a/src/png.ts
+++ b/src/png.ts
@@ -216,3 +216,5 @@ export class PNG extends EventEmitter {
     PNG.adjustGamma(this)
   }
 }
+
+export default PNG